AN 



INTRODUCTION 

ASTRONOMY 



JJKSIGNED AS A 



READER AND TEXT-BOOK 



SCHOOLS AND FAMILIES. 



Rev. JOHN T)AA^IS, A.M 

FOKMERLY PEOFESSOR OF MATHEMATICS AXD ASTRONOMY IN 

ALLEGHENY CITY COLLEGE, AND LATE PRINCIPAL OF 

THE ACADEMY OF SCIENCE, ALLEGHENY CITY, PA. 




PHILADELPHIA: 
J. B. LIPPINCOTT & CO. 

1868. 



Entered, according to Act of Congress, in the year 1867, by 

JOHN DAYIS, 

in the Clerk's OflBce of the District Court of the United States for the Western 

District of Pennsylvania. 



STEREOTYPED BY MACKELLAR, SMITHS & JORDAN, 
PHILADELPHIA. 



<; 



\^ 




CAXTON PRESS OF 
;RMAN & CO., PHILADELPHIA 



PREFACE. 



This work is designed to fill a vacuum in schools 
and families. With the advancement of science there 
should be a corresponding advancement in the facilities 
for acquiring a knowledge of it. To economize time 
and expense in this department is of as much import- 
ance to the student as frugality and industry are to the 
success of the manufacturer or the merchant. Im- 
pressed with the importance of these facts, and having 
a desire to aid in the general diffusion of useful know- 
ledge by giving them some practical form, this work 
has been prepared. It has a double purpose. As a 
reader, it contains a great variety of topics ; and, being 
almost entirely divested of scientific terms and phrases, 
its language is level to the comprehension of the youthful 
mind. As a text-book, it treats first of the sun and 
those heavenly bodies with which we are all, by obser- 
vation, most familiar, and advances, consecutively, in 
the investigation of other worlds and systems which 
the telescope has revealed to our view. Thus to enhance 
the interest and value of this work to the student and 
casual reader, nearly all of the topics that it contains 
are fully illustrated by engravings, prepared expressly 
for that purpose. And to promote this object still 
further, and impart knowledge in the most impressive 
manner by a sensible demonstration of the arrangement, 
relations, and various motions of the component parts 



4 PREFACE. 

of the planetary system, the Planetelles, Heliotellus, 
and Lunatellus, have been invented by the author and 
put into public use. Each of these instruments is in- 
valuable in acquiring a knowledge of astronomy and 
geography. With these facilities for illustration which 
address themselves to the mind through the eye, and the 
plainness and simplicity which characterize the entire 
work, the author would respectfully present it to a gene- 
rous public, trusting that it may be the means of dis- 
seminating a general knowledge of that ennobling 
science on which it treats. 



CONTENTS. 



PAaE 

Planetelles 11 

Heliotellus 13 

luxatellus 15 

Section I. — History of Astronomy 17 

II.— The Planetary or Solar System 19 

TIL— The Sun 23 

IV. — Sun's Atmosphere 21 

v.— Solar Spots 25 

VL— Motions of the Sun 2G 

VII.— Mercury 29 

VIII.— Venus 33 

IX. — Transits, Conjunctions, and Motions of Mercury 

and Venus 34 

X. — The Earth, — its Annual Motion 36 

XL— Daily Motion of the Earth 38 

XII.— The Moon 40 

XIII. — Phases of the Moon 41 

XIV. — Solar and Lunar Eclipses 42 

XV. — Lunar Eclipses 45 

XVI. — Lunar Influences 45 

XVII. — Lunar Atmosphere 48 

XVIIL— Lunar Surface 50 

XIX.— Mars 54 

XX.— The Asteroids 55 

XXI. — Jupiter 58 

XXII. — Jupiter's Moons 60 

XXIIL— Saturn and his Rings 62 

XXIV.— Saturn and his Moons 66 

XXV. — Uranus, Herschel, or Georgium Sidus 68 

1* 5 



6 CONTENTS. 

PAGE 

Section XXYL— Satellites of Uranus 69 

XXVIL— Neptune 70 

XXVIIL— Satellites of Neptune 71 

XXIX.— Light and Heat on Neptune 72 

XXX.— The Earth— one of a Class 74 

XXXL— Planets inhabited 75 

XXXII. — General Configuration of the Heavenly Bodies 77 

XXXIIL— Specific Form of the Earth 79 

XXXI v.— The Planets— Oblate Spheroids 81 

XXXV. — Position of the Solar System in Relation to the 

Starry Heavens 82 

XXXVI.- -Motions of the Planets around the Sun 85- 

XXX VII.— Centrifugal and Centripetal Forces 87 

XXXVII L— Motion of the Planets on their Axes 90 

XXXIX.— Weight of Objects on the Sun and Planets 91 

XL. — Earth's Atmosphere 93 

XLL— Eefraction 96 

XLIL— Twilight 99 

XLIIL— Aurora Borealis 101 

XLIV.— Shooting Stars 105 

XLV. — Meteorites, Aerolites, and Fireballs 108 

XLVL— Zodiacal Light 112 

XLVIL— Light 115 

XLVIIL— Colors of Light 119 

XLIX.—Eefracting Telescope 123 

L. — Reflecting Telescope 125 

LL— Comets— Their Early History 127 

LIL— Encke's Comet 129 

LIII.—Biela's Comet 132 

LIV.— Halley's Comet 135 

LV.— The Comet of 1744 137 

LVL— The Comet of 1843 140 

LVII. — General Form and Appearance of Comets 142 

LVIIL— Gravity of Comets 144 



CONTENTS. 7 

PAGE 

Section LIX. — Earity of Comets 145 

LX.— Nature of Comets 148 

LXI.— Comets shine by Keflected Light.. 149 

LXII. — Anomalous Aspect of Comets 152 

LXIIL— Tails of Comets— How formed 154 

LXIY. — Superstitious Notions concerning Comets 157 

LXV.— Distance to Fixed Stars 159 

LXVL— Classification of Fixed Stars 161 

LXVIL— Stars have no Sensible Discs 164 

LXVIII.— Eeal Magnitude of Stars 165 

LXIX. — Method of computing Distances to Fixed Stars 

by Trigonometry 167 

LXX. — Telescopic Method of computing Distances of 

Heavenly Bodies 168 

LXXI. — The Stars are Self-Luminous Bodies 171 

LXXIL— Color of Fixed Stars 174 

LXXIII. — New Temporary Stars 175 

LXXIV. — New Permanent and Periodical Stars 177 

LXX v.— Eelations of Multiple Stars 179 

LXXVL— Stars Optically United 179 

LXXVIL-Stars Physically United 181 

LXXYIIL— Clusters of Stars 183 

LXXIX.-Nebulffi 186 

LXXX.— The Milky Way 189 

LXXXL— Magellan Clouds 190 

LXXXIL— Nebula Proper 191 

LXXXIIL— Annular Nebulffi 193 

LXXXIV.— Stellar Nebula?, and Nebulous Stars 195 

LXXXV.— Planetary Nebulffi 197 

LXXXVL— Globular Nebulse or Clusters 199 

LXXXVn.— Spiral Nebula 201 

LXXXVni.— Structure and Harmony of the Universe 203 

Defixitioxs of Astroxomicae Ter:^is and Phrases 333 



P LANETELLE S. 

The Planetelles, which is represented by Fig. 1 on the opposite 
page, was invented by the author of this Avork, and has been in 
the course of construction for more than twenty years. Its first 
conception originated in the conviction that if the solar system, 
constituted as it is of many parts, and all of these parts having various 
motions, could be represented by an artificial system like itself, exhibit- 
ing the same phenomena in the same order, aid of incalculable 
value would be furnished in the acquisition of knoAvledge. With the 
view of rendering this idea practical, the construction of a machine 
for that purpose was commenced, and pursued, till the otie which is 
here represented was completed. This instrument represents the 
motion of the sun on his axis, the relative annual motions of the 
eight primary planets, also their relative diurnal motions, and the 
various motions of the moon and of all the satellites around their 
primaries in difierent periods. It shows the relations existing be- 
tween the sun and all the primary planets, and their satellites and 
each other, and is designed to illustrate, also, the phenomena result- 
ing therefrom. It presents to the eye eighty difierent motions of 
these bodies, and illustrates the succession of day and night on each 
of the primary planets, the inclination of their axes to the planes of 
their orbits, the change of their seasons, the amount of change in 
the sun's declination in relation to each, and his rising and setting on 
each, the difierent lengths of their days and nights, the difierence in 
the length of their seasons, their conjunctions, and the phases of 
Mercury and Venus, the retrogression of the moon's nodes, and the 
length of lunar days and nights, the solar and lunar eclipses, and the 
changes, phases, fulling, and eclipses, of all the satellites. 

All these phenomena, besides a vast amount of other useful know- 
ledge, may be explained by the use of this instrument and the arti- 
ficial zodiac that is connected with it, so that even the youthful and 
uneducated mind cannot fail to understand the astronomy of the solar 
system, if disposed to give the subject any attention. No mechanical 
device that has ever been brought before the public presents such an 
ocular demonstration of the planetary system in motion, and expresses 
its phenomena Avitli such precision and accuracy, as the Planetelles ; 
and as it is portable, occupies but little space, and is not liable to get 
out of order, is in every way adapted to the private study and the 
student's class-room, — places which it fills, as an illustrator of science, 
with greater fidelity, in manv respects, than the living instructor. 
10 



11 




Fig. 1.— Planetelles. 



HELIOTELLUS. 



The Heliotelltjs, which is represented by Fig. 2 on the opposite 
page, is constructed on the same principle on which the Planetelles 
is, and is designed in part for the same purpose. It contains five 
globular bodies, which, by their arrangement, relations, and motions, 
exhibit the arrangement, relations, and motions of the sun, Mercury, 
Venus, Earth, and the moon, as so many parts of the solar system. 
This instrument represents the motion of the sun on his axis, the 
relative yearly motions of Mercury, Venus, and the earth, ako their 
relative daily motions, and the motions of the moon around the earth, 
around the sun, and on her axis. It shows the inclination of the axes 
of Venus and the earth to the planes of their orbits, and is designed 
to illustrate the respective widths of their difierent zones, the succession 
and duration of their different seasons, the different lengths of their 
days and nights, the amount of change of the sun's declination in 
relation to each, and his rising and setting on each, the superior and 
inferior conjunctions and phases of Mercury and Venus, also the solar 
and lunar eclipses, the length of lunar days and nights, the retro- 
gression of the moon's nodes, and the changes, phases, and fulling of 
the moon . 

By the use of the Heliotellus and the artificial zodiac connected 
therewith, these phenomena, the tides also, and other elements of 
useful knowledge, can be explained with such clearness and sim- 
plicity that even a child, with little attention, cannot fail to com- 
prehend them. This instrument, for the purpose of illustrating a 
portion of the planetary system and the phenomena resulting there- 
from, is invaluable ; and, as it can be carried in the hand, and occupies 
but little space, economizes the time and expense of the pupil, and 
diminishes the labor of the teacher, is especially adapted to the use 
of families, and every grade of schools in which the elements or 
more advanced branches of Astronomy or Geographv are taught, 
]2 



13 




Fig. 2 — Heliotellus, 
2 



LUNATELLDS. 



The LtjnatelLiUS, which is represented by the cut on the opposite 
page, illustrates the astronomical phenomena of the sun, earth, and 
moon in their natural order, with the geography of the earth in its 
proper relation to them. The sun is made to turn on his axis, the 
earth revolves at her proper inclination on her axis and around the 
sun, producing not only the change of seasons and the vicissitudes 
of day and night, but also their natural increase and decrease in 
length. The moon revolves around the sun, on her axis, and around 
the earth, producing the alternation of her days and nights, her 
changes, phases, and fulling, and also, by her retrograding at every 
revolution, the interesting phenomena of solar and lunar eclipses. 
And as the natural divisions of the surface of the earth and the 
political divisions of every country are marked on the body that is 
employed to represent the earth, the geographical and astronomical 
relations of every locality, at any particular period during the year, 
are clearly exhibited to the eye. 

The daily use of the Lunatellus by both teacher and pupil has 
clearly demonstrated that, in addition to its advantages in the study 
of astronomical phenomena, it is almost indispensable in acquiring 
an accurate knowledge of Descriptive and Physical Geography, as the 
bodies that represent the earth and the moon occupy their relative 
places and positions in relation to the sun and each other, and can 
have given to them at pleasure their relative motions in their rela- 
tive periods. 

14 



15 




LTinatelliis. 



INTRODUCTION TO ASTRONOMY. 



SECTION I. 



Pbtorg of gistronomi). 

1. Astronomy is the most ancient of all the sciences. 
The first astronomers were shepherds, and some of their 
observations date back more than twenty-four hundred 
years before Christ. Job, who was a native of Chaldea 
during its early history, speaks of Orion and the 
Pleiades, — certain groups of stars which still retain 
their former names. Homer and Hesiod also had a 
knowledge of at least some of the divisions of the 
starry heavens, Avhich had been made; and Aratus 
enumerated nearly all of the ancient constellations.^ 

2. It is highly probable that the Chaldeans and 
Egyptians were the first to draw boundary lines of the 
richest stellar districts and designate them by certain 
names. The Greeks and Romans did the same ; and to 
these nations are we indebted for at least some of the 
divisions of the heavens which we find noted in our 
astronomical maps. 

3. To Avhat extent the Chaldeans and Egyptians 
carried their divisions, or where the Greeks and Romans 
took the subject up, is uncertain ; yet it is evident from 
their records that not only they, but the Chinese also, 
had a knowledge of many of the prominent constella- 



^ Constellations — are groups of stars. 



Questions. — 1. "What is the most ancient science? AVho were the 
first astronomers? 2. Wlio first divided the starry heavens into con- 
stellations? Who next? 3. What other nation had a knowledge of 
them ? 

2* 17 



18 HISTORY OF ASTRONOMY. 

tions in both hemispheres/ as well as those that con- 
stitute the zodiac.^ 

4. Thales, one of the seven wise men of Greece, who 
lived six hundred years before the Christian era, was 
the first regular teacher of astronomy. Pythagoras, 
also of Greece, shortly afterwards taught this science 
in a celebrated school at Crotona, and was the first to 
discover the true theory of planetary motion ; but did 
not succeed in having his views adopted, owing to the 
ancient prejudices and incredulity of those who regarded 
themselves as being pre-eminent in science and discovery. 

5. Hipparchus — about three hundred years before 
the Christian era — distinguished himself as a teacher 
of astronomy in the most celebrated school in Egypt; 
and Ptolemy, of the same nation, one hundred and 
seventy years afterwards, acquired still greater celebrity. 
He devised a theory from what was merely apparent in 
the heavens, regarding the earth as stationary and the 
centre of the universe, by which he endeavored to ac- 
count for nearly all the changes and phenomena which 
we discover in the solar system. 

6. His opinions were entertained and adopted by the 
various schools as being most consonant with what pre- 
sented itself at different times in the heavens to ^he 
senses, and, in consequence of its resemblance to the 
truth, continued to be taught in the academies and 
schools, till Copernicus, of Prussia, revived, in fifteen 
hundred and thirty, the Pythagorean or true theory of 
planetary motion, viz. : — that the sun is the central orb 
of the system, and that all the planets revolve around 



^ Hemisphere — is half a sphere. 

^Zodiac — is twelve constellations which form a belt of stars, 16° 
wide, clear round the heavens. 

Questions. — 4. Who were the first teachers of astronomy ? Who 
discovered the true theory of planetary motion? 5. What distin- 
guished astronomers succeeded Thales and Pythagoras? Whose 
astronomical theory was taught in the schools from the second till 
the sixteenth century ? 6. Whose theory was revived by Copernicur,, 
in fifteen hundred and thirty ? 



THE SOLAR SYSTEM. 19 

him. He discussed the true theory of the celestial mo- 
tions, distinguishing between what was real and what was 
only apparent, and thereby rescued tliis science from the 
doubt and uncertainty which had always surrounded it. 

7. Tycho l^rahe, of Denmark, also lent invaluable aid 
to discovery, in the use of those instruments which were 
then employed in astronomical observation; and Kepler, 
a German astronomer, discovered the three great laws 
of planetary motion. So also did Galileo, with the aid 
of the telescope, make new discoveries which were higlily 
interesting; and Sir Isaac Newton saw and made known 
the application of that grea|/ principle in tlie law of gra- 
vitation, whereby all worlds and heavenly systems are 
governed and controlled." 

8. And to more fully develop and more firmly esta- 
blish the facts already obtained, many studious observers 
are still engaged, in the Old World and the New, in ex- 
ploring and investigating the motions and movements 
of those celestial bodies wdiich are silently satisfying the 
will of their Creator in the economy of nature. 



SECTION II. 

^laiutarjj or ^olar ^jistcin. 



1. This system of worlds and heavenly bodies is the 
one to which our earth belongs, and is now known to 
contain eight primary planets,^ eighty-five asteroids^ or 
smaller planets, twenty-one satellites,^ moons, or second- 

^ Planet — a wanderer, 
2 Asteroid — a small planet. 

^ Satellites, moons, or secondary planets, are those that revolve 
around the primary planets. 

Questions. — 7. What celebrated astronomer succeeded Copernicus? 
Who applied the principle of gravitation to the heavenly bodies ? 
8. Are astronomers now engaged in the study of this science ? 

Sec. II. — 1. How many primary planets are there? How many 
asteroids? How many satellites, moons, or secondary planets? Are 
there manv comets? 



20 



THE SOLAR SYSTEM. 




Fig. 3.— Solar System, 

ary planets, a great number of comets, and the sun also, 
which is the controlling centre of them all. 

2. The primary^ planets are of different magnitudes, 
and revolve around the sun in different periods and at 
different distances from him. They are all known by 

* Primary planet — a large planet. 



Questions. — 2. Are the primary planets alike in magnitude? Are 
tliey at different distances from the sun ? Do they all revolve around 
him in the same length of time ? Are they known by certain names? 
What were they named after? What are the names of the two 
nearest the sun ? 



THE SOLAR SYSTEM.. 21 

certain names, which are derived from the names of 
heathen gods or objects of idolatrous worship. Mercury, 
which is the planet nearest the sun, was named after the 
god of dishonesty and injustice; aud Venus, the second 
planet from the sun, was named after the goddess of love 
and beauty. 

3. Mars, the fourth planet from the sun, received his 
name from the god that was supposed to rule over war 
and battle; and Jupiter, the fifth planet in point of dis- 
tance from the sun, received his name from the name 
of the imaginary being that was supposed to be god 
over all. Saturn, the sixth planet from the sun, ob- 
tained his name from the imaginary being who was 
supposed to preside over time and chronology; and 
Uranus, the seventh planet from the sun, obtained his 
name from the god of astronomy. Neptune, the outer- 
most planet from the sun, was named after the imaginary 
deity that was supposed to preside over the seas. 

4. The asteroids are small planets that revolve around 
the sun as their centre of motion. They travel around 
him in orbits^ which lie between those of Mars aud 
Jupiter. Nearly all of them are invisible to the naked 
eye; and had it not been for the aid of the telescope they 
would have remained undiscovered. 

5. The length of their annual periods is nearly the 
same, each being about the length of four and one-half 
of our years. Their average distance from the sun is 
about two hundred and sixty-one millions of miles. 
Like the primary planets, they are all known by certain 
names, which were given to them generally by their 
discoverers, and will be noticed hereafter. 

6. The secondary planets are small bodies that revolve 

^ Orbit — the path of a planet round the sun. 

Questions. — 3. What are the names of all further away from the 
sun than the earth? 4. "What do the asteroids revolve around? 
Between the orbits of what two planets do they revolve ? Are they 
visible without the aid of the telescope ? 5. What is the average 
length of their years? What is their average distance from the 



22 THE SOLAR SYSTEM. 

around some of the primary planets as they travel around 
the sun. Our moon is one of them, and the only one 
that revolves around the earth. Jupiter, the fifth planet 
from the sun, has four of them, which revolve around 
him in different periods. The one nearest to him moves 
fastest, and has the shortest period. The second moves 
slower than the first, and has a longer period. In 
like manner is it with the third and fourth. 

7. Saturn, the sixth planet from the sun, has eight 
moons or satellites that revolve around him. They are 
at cliiferent distances from him, travel with different 
velocities, and have different periods. They follow the 
order of all heavenly bodies in their movements that have 
a centre of motion, diminishing in their velocities and 
increasing in their periods as their distances increase 
from their primaries. 

8. Herschel, or Uranus, the seventh planet from the 
sun, has six moons, which are at different distances from 
him and revolve around him with various velocities and 
in different periods. Neptune, the outermost planet, has 
two moons, which are similar to those of the other 
planets which we have noticed, in their distances, move- 
ments, and length of their periods. 

9. The comets are rare bodies which revolve around 
the sun, and are seldom seen except when they'^are near 
to him. Several hundred of them have been discovered 
with the aid of the telescope, and the motions and ele- 
ments of the orbits of many of them have been pretty 
accurately computed. Thongh some of them are very 
large, they have very little matter in them, as has been 
fully demonstrated by the slight attraction which they 
have for other bodies that are sometimes comparatively 
near to them. 

10. All of these heavenly bodies to which we have 

Questions. — 6. What do the secondary planets revolve around ? 
Is the moon a secondary planet ? How many has Jupiter ? 7. How 
many has Saturn? 8. Hoav many has Herschel, or Uranus? How 
many has Neptune? 9. Are comets rare or dense bodies? Are 
they large or small? Around what body do they revolve? 



THE SUN. 23 

in this section referred are objects of special interest, and 
we will notice them in their order of distance from the 
sun, making him the subject of our first remarks. 



SECTION III. 



-Telescopic View of tlie Sim. 

1. The sun, which is the central orb of our system, 
being the most prominent of all the heavenly bodies, 

Qhestioxs.— 1. What distance is it to the sun? What is his 
diameter? How much larger is the sun than all the bodies that 
revolve around him ? How much heavier ? 



24 sun's atmosphere. 

claims now our immediate attention.^ He is ninety-five 
millions of miles distant from us, and nearly nine hun- 
dred thousand in diameter, and his axis is inclined about 
7° to the plane of the ecliptic. His volume is six hun- 
dred times greater than that of all the bodies that revolve 
around him, and in mass or weight he is about seven 
hundred greater. 

2. He is fourteen hundred thousand times larger than 
the earth, and his density is nearly one-fourth that of 
the earth, and a little greater than that of water. He 
revolves on his axis-^ in about twenty-five and one-half 
days, and is the great source of light and heat, which 
enlightens and gives warmth to all of the pLanets and 
bodies that revolve around him. 



SECTION IV. 

Mini's gitmospbrn. 

1. The sun is nearly globular in form, and appears 
to be invested with three envelopes or atmospheres, dif- 
fering in their nature and densities. The one next to 
his body seems to be in a measure transparent, sustain- 
ing cloudy matter in its upper regions, sinjilar to the 
clouds which are suspended in our own atmosphere. 
This opinion is entertained from the different shades of 
light and darkness which are associated with the spots 
which frequently appear on his disk.^ 

2. The second envelope, or atmosphere, is supposed to 

^ Axis — an imaginary line passing through the centre of a body 
from one pole to the other. 
"^ Disk — the face of the sun. 

Questions. — 2. How much larger is he than the earth ? Is he 
as dense as the earth ? Is he as dense as water ? Does he revolve 
on his axis ? In what time ? 

Sec. IV. — 1. How many atmospheres are supposed to surround 
the sun ? What is the nature of the inner one ? 

* For purposes of convenience, round numbers are generally em- 
ployed in this work to express the magnitudes, distances, and velo- 
cities of the henvenlv bodies. 



SOLAR SPOTS. 20 

be the great reservoir of solar light and heat, and the 
place where they are generated. From experiments 
which have been made recently, it has been discovered 
that the light which is reflected or emitted from a solid or 
liquid substance heated to a certain temperature differs 
from what is emitted from a gaseous or ethereal substance. 

3. The sunlight is of the latter kind; which would 
indicate that his solid body is not its source. This light 
and heat may be produced by the agency of electrical 
currents without combustion, as it is not always essen- 
tial to their production. 

4. The existence of a third or outer envelope, con- 
sisting of very attenuated matter, is insisted on by some 
astronomers, from tlie peculiarities attending the sun 
when he is totally eclipsed. Thin cloudy matter was 
observed at and beyond his margin, and columns of 
rose-colored light ascended sometimes from it to the 
height of forty or fifty thousand miles, and would then 
move off in a horizontal direction. 

5. These phenomena are indicative of the existence 
of a gaseous substance, capable of sustaining visible 
matter, and more remote from the solid solar globe 
than his photosphere.^ 



SECTION V. 

^olar §jjots. 

1 . The spots which frequently aj^pear on the sun are 
supposed to be openings in his luminous atmosphere — 

^ Photosphere — the spherical gaseous matter around the sun's body, 
in which his light is produced. 

QuESTioxs. — 2. Where is the sunlight supposed to he generated ? 
Does the light that is emitted from a gaseous or ethereal substance 
differ from what is emitted from a solid or liquid substance ? 3, 
What kind of light is sunlight ? What agency may produce his 
light and heat? 4. What is the nature of the sun's outer atmos- 
phere ? 5. What evidence is there of its existence and nature ? 

?> 



26 MOTIONS OF THE SUN. 

produced probably by whirlwinds beneath, or other dis- 
turbing causes originating in their own vicinity. They 
are seldom seen within three degrees of the sun's equa- 
tor, and never at his poles. 

2. The atmosphere of the earth, at certain elevations, 
is in a far greater state of agitation in zones that are on 
either side of the equator than elsewhere; and may it 
not be so in similar regions on the sun? His whole 
surface appears, when viewed by the telescope, to be in 
a state of constant agitation, the agitation being always 
greatest where the spots break out, and their funnel- 
shaped forms, with other attending circumstances, show 
that they are not scoriae nor any solid substance. 

3. Some of them at times are forty or fifty thousand 
miles in diameter; and sometimes so many of them have 
been on the sun's disk as to diminish his light and heat 
at least one per cent. These spots are not only variable, 
but periodical, increasing in number for five and one- 
half years, and decreasing for the same period, corre- 
sponding exactly in point of time with a certain varia- 
tion observed in the intensity of terrestrial magnetism. 
See Fig. 4. 



SECTION VI. 

Potions of t^e 3mx. 

1. The sun has three motions, — one around the centre 
of gravity of the planetary system, the second on his 
own axis, and the third around the centre of this stellar 

Questions. — 1. What are the spots on the sun supposed to be ? 
What may cause them ? Are they observed near his equator, or his 
poles ? 2. Where is the earth's atmosphere, at certain elevations, 
agitated most ? How does the sun's surface appear when viewed 
with the telescope? What evidence is there that these spots are 
not composed of solid substance ? 3. What is the diameter of some 
of these spots ? Are they ever so numerous as to diminish the sun's 
light and heat ? Do these spots change in size ? Are they periodical ? 
What is the length of their periods ? With what do they correspond 
that belongs to the earth ? 

Sec. VI. — 1. How many motions has the sun ? 



MOTIONS OF THE SUN. 




Pig. 5. 



28 MOTIONS OF THE SUN. 

universe, supposed to be at or near the star Alcyone in 
the Pleiades. 

2. By Fig. 5 we will illustrate the first of these 
motions. If all of the planets with their satellites were 
attached to a rod at their relative distances from the sun, 
and the sun himself attached to the other end, and the 
rod placed on a fulcrum at the point where the sun 
would balance or hold in equilibrium all the other 
bodies, that point would be the centre of gravity of the 
whole system. 

3. By revolving this rod, all of these bodies would 
revolve around this centre; and, the sun being hundreds 
of times heavier than the aggregate of all the rest, he 
would be always nearest to it, even so near that it would 
generally be between his surface and his centre. This 
centre of gravity constantly changes a little in position, 
in consequence of the changes which the planets make 
in their orbits in performing their annual revolutions. 

4. Besides the motion of the sun around the centre 
of gravity of our system, he has a motion from west to 
east on his axis. This fact is known by observing the 
spots on his disk. They always pass over his surface in 
one general direction, exhibiting at the same time all 
those changes of form which would result from the 
revolution of a globular body on its axis. The time 
they occupy in passing from one side of his disk to the 
other indicates that it requires nearly twenty-five and a 
half days for him to make one rotation. 

5. Another motion of the sun is around the centre 
of gravity of this stellar universe, to which our whole 
system belongs. The most familiar evidence that we 
have of this motion is founded on a fixed principle 

Questions. — 2. If the sun and planets were fastened to a rod at 
relative distances from each other, where would be their centre of 
gravity ? 3. Around what would they revolve if put in motion ? 
What causes this centre of gravity to change its place a little ? 
4. Does tlie sun revolve on his axis ? In what direction is this ro- 
tation ? How is this known ? 5. Does the sun himself revolve 
around a centre, like the planets ? What is the evidence that he has 
a planetary motion founded on ? 



MEECURY. 



29 



known to us all, — viz. : that objects at a distance appear 
to diverge and increase in size the nearer we approach 
them, and to converge and diminish the further that we 
recede from them. By a series of observations, this has 
been discovered to be true in relation to the sun. 

6. Along the imaginary track over which he has tra- 
velled, the space between the heavenly bodies appears 
to have diminished; while in the opposite direction the 
space between the stars in the constellation Hercules 
appears to have increased. From these coincidents and 
other attending circumstances, it is inferred that he is 
not only travelling like a planet in its orbit, attended by 
a retinue of worlds which are controlled by him, but that 
he will be more than eighteen millions of years in making 
one of his annual revolutions. 



SECTION VII. 



1. Mercury is the name of the planet nearest the sun, 

is about thirty-one 
hundred miles in 
diameter, and is 
one - sixteenth the 
size of the earth and 
nearly three times 
as dense. He makes 
I a revolution around 
I the sun in about 
eighty -eight days, 
is thirty-seven mil- 
lions of miles dis- 
tant from him, and 
moves in his orbit 
with the amazing 

Fig. 6 -Telescopic View of Merciuy in Quadratuxe. Velocity of one hun- 
dred and nine thou- 




QuESTiON. — 6. What is the evidence itself? 
3* 



30 



MERCURY. 



sand miles per hour. His days and nights are nearly 
the same length as ours ; and it is unknown if he has 
a change of seasons. 

2. He is an opaque^ body, as all the bodies are that 
revolve around the sun, and he is seldom visible, and 
when so only in the twilight, owing to the proximity 
of his orbit to the sun. The light and heat which he 
receives from the sun are six and one-half times more 
than what we receive, if their intensities on his surface 
are regulated by that law wdiich obtains concerning 
them. The law regulating light is illustrated by Fig. 7. 

3. If this first opening is one linear foot from a 





1) 


.^-"'''^ 


1 ffiSIBicil 



Fig. 7. 
luminous body, and its area^ one square foot, the light 

^ Opaque body — one that has no light of its own. 
2 Area — means surface generally; sometimes it means a hole or 
opening. 

Questions. — 1. What planet is nearest the sun? What is his 
size when compared with the earth ? What is his diameter ? What 
is the length of his yea];? What is his distance from the sun? 
What is said of his days and nights ? 2. Does he shine with his ewn 
light ? and when can lie be seen ? How much more light and heat 
does he receive than we, according to the law which obtains con- 
cerning them? 3. How much more feeble is light two feet from 
the source of light tlian one foot from it ? — three feet than one foot 
from it ? 



MERCURY. 31 

which passes through it will cover an area of four square 
feet two feet from the source of light, and cover nine 
square feet three feet from the body that emits the light. 
The intensity of the light is nine times less three feet 
from the source of light than one foot from it; which 
makes it evident that its increase and decrease of inten- 
sity are regulated by a definite law. 

4. Heat and sound, the force of attraction, and the 
influence of magnetism, follow this same law. They 
diminish in an inverse ratio as the distance increases : 
consequently, the definite amount of heat and light 
which each planet receives can be ascertained if its dis- 
tance from the sun is known, unless there should 
be some modifying agencies connected w^ith the planets 
themselves, which we will have occasion to notice here- 
after. 

5. The physical constitution of Mercury is probably 
much the same as the physical constitution of the earth. 
His appearance evinces that he has an atmosphere which 
may answer not only a similar purpose to our own, but 
also shield him from that insufferable light and heat 
Avith which he is constantly surrounded. 

6. He passes through various phases,^ like those of 
the moon as she makes her revolutions around the earth, 
exhibiting at various periods less and more of an en- 
lightened surface. These phases are common to both 
Mercury and the next outermost planet, Venus, and 
serve to establish the fact that their orbits are nearer to 
the sun than the earth's, and that they shine by borrowed 
light. The various portions of enlightened surface of 
these bodies when in different points of their orbits 
mav be seen bv referrino; to Ym. 8. 



^ Phases — Different amounts of enlightened surface visible. 

Questions. — 4. What other agents conform to this law ? Is it 
necessary to know the distance to a planet that we may know the 
amount of light and heat that it receives ? 5. Does Mercury appear 
to have an atmosphere? What purposes may it answer? 6. Has 
he phases like the moon ? Has Venus the same phases ? What do 
they prove? 



32 



MERCURY. 



7. Suppose the body in the centre to be the sun, and 
the first from him Mercury, the second Yenus, the third 
the earth, and the fourth Mars. If Mercury and Yenus 
are on the opposite side of the sun from us, we will 
see one-half of their surfaces enlightened. Move them 
in either direction, and we will see less of their surfaces 



MARS 




i 



enlightened, until they become invisible, when they 
arrive at that point where they are between us and the 
sun. 

8. These planets present such changes in relation to 
their light; and were it possible that Yenus could have 
that half of her surface next to us enlightened when at 



Questions. — 7. When do Mercury and Venus present to us the 
greatest amount of enlightened surface? When are they invisible? 



VEXUS. 



33 



her nearest point to us, she would appear twenty-five 
times larger than she does. But, as she recedes from us, 
her hither^ hemisphere becomes more and more enlight- 
ened; and, these changes occurring simultaneously, she 
appears to vary very little at any time in her size. 

9. These phenomena- are never manifested by those 
planets Avhich are more distant from the sun than the 
earth. They, being constantly outside of the orbit of 
the earth, always exhibit nearly the whole of their en- 
lightened sides, which are next to us, as manifested by 
Mars, in Fio;. 8. 



SECTION VIII. 

1. Venus 
is the second 
planet in 
point of dis- 
tance f r m 
thesun,which 
is apparent 
from the fact 
that she is 
always seen, 
when visible, 
at a greater 
distance from 
him than 
Mercury. — 
Her mean 
distance fi^om 
the -sun is ^'^S- 9.— Telescopic View of Venus after Inferior Conjunctioi:. 




^ Hither hemisphere — the one next to us. 

2 Phenomena — natural characteristics or changes in appearance. 

Questions. — S. 'Why does Venus appear to vary yery little in 
her size ? 9. Do the planets outside of the earth exhibit phases 
like Mercury and Venus ? "Why do they not ? 



34 TRANSITS OF MERCURY, ETC. 

sixty-eight millions of miles, and h^r magnitude is one- 
seventh less than that of the earth. She moves in her 
orbit with a velocity of eighty thousand miles per hour, 
and rotates on her axis in a little less than twenty-four 
hours. 

2. She appears to have an atmosphere in which is 
suspended at times cloudy matter, and by the marks 
visible by the telescope the period of her daily rotation 
is determined. As the morning star she rises for a 
period of two hundred and ninety-two days before the 
sun, and as the evening star sets after him for the same 
length of time. 

3. Unlike all of the other planets, she has eight 
seasons,^ — two springs, two summers, two autumns, and 
two winters, at her equator; and four at her tropics,^ — 
spring, summer, autumn, and winter. This unusual num- 
ber of seasons occurs in consequence of the great obliquity 
of her axis to the plane of her orbit, which renders her 
polar circles only fifteen degrees from her equator, and 
her tropics only fifteen degrees from her poles. 



SECTION IX. 

i^ransits anb Conjunttions of P^ercurg mxh WtnuQ. 

1. Venus, like Mercury, makes transits ^ over the 
sun's disk, passing like a little black spot from one side 
of him to the other, indicating thereby that these bodies 



^ Tropics — on the terrestrial sphere, the circles that divide the 
torrid from the temperate zones. 
2 Transit — passage across or over. 

Questions. — 1. What is the distance of Venus from the sun? 
How much less is she than the earth ? At what rate does she travel 
in her orbit ? Does she rotate on her axis ? 2. What appears to 
be in her atmosphere ? How long is she the morning star ? How 
long the evening star ? 3. How many seasons has she at her equa- 
tor? How many at her tropics ? What causes this unusual number 
of seasons ? 



TRANSITS OF MERCURY, ETC. 35 

are not only inferior to the earth in position, but that 
they shine by borrowed light. The transits of these 
bodies cannot occur very frequently, owing to the fact 
that the planes of their orbits do not coincide with the 
plane of the ecliptic.^ 

2. Their planes cross the plane of the ecliptic at 
different angles, and, consequently, transits can only 
occur when the sun and planets are near these crossing 
points or nodes. Two of these nodes belong to each 
orbit ; and as these planets pass from the north side of 
the ecliptic to the south, they pass through their descend- 
ing nodes, and as they return from south to north, they 
pass through their ascending nodes ; and the imaginary 
line that extends from one node to the other is called 
the line of nodes. 

3. When Mercury transits the sun, it will be in May 
or November ; and when Venus transits him, it will be 
in June or December. 

4. Mercury and Venus when on the side of the sun 
next to us are in their inferior conjunctions, and when 
on the opposite side of the sun from us are in their 
superior conjunctions. 

5. Their motions are said to be direct and retrograde : 
direct when they are passing through their superior con- 
junctions, and retrograde when passing through their 
inferior conjunctions. 

6. These planets are termed inferior, in consequence 
of their orbits being nearer to the sun than the orbit 
of the earth. 



^ Ecliptic — the great circle whicli the sun appears to describe 
annually among the stars. 

Questions. — 1. What is the transit of a planet? How many of 
the planets transit the sun ? What do their transits prove ? Can 
they occur frequently ? Why ? 2. When do transits occur ? When 
are these planets said to pass through their ascending and descending 
nodes ? 3. In what months do Mercury and Venus transit the sun ? 
4. Where are Mercury and Venus when in inferior conjunction ? 
Where when in superior conjunction ? 5. When are their motions 
direct ? When retrogade ? 6. Why are these planets called inferior ? 



36 



THE EARTH. 



SECTION X. 

€\n ^artlj— Us g^mttial Potion. 

1. The earth resembles an orange in form, being 
flattened at its poles, making the distance between the 
poles twenty-six miles shorter than the distance from 
the equator on one side to the equator on the other. It 
is surrounded with an atmosphere about forty-five miles 
in depth, and has four seasons in its temperate zones ; 
and two, summer and winter, about its poles. 

2. Its diameter is 
nearly eight thou- 
sand miles, and, con- 
sequently, its cir- 
cumference is about 
twenty-five thou- 
sand. Its mean dis- 
tance from the sun is 
ninety-five millions 
of miles, and it re- 
volves around him 
in one year, and also 
rotates on its axis in 
twenty-four hours. 
Notwithstanding 
these declarations in 
relation to its mo- 
tions are true, they were not only in obscurity in ancient 
times, but were received with much distrust by many 
at a much later date. 

3. Every system of astronomy, as well as almost 
every astronomer, anterior to the revival of letters in 




Fis, 10. 



Qtjestioxs. — 1. How mucli shorter is the polar than the equato- 
rial diameter of the earth ? What is the distance to the surface of 
the atmosphere ? How many seasons in the temperate zones ? How 
many at the poles? 2. "What is the diameter of the earth? What 
is its circumference ? What is its distance from the sun ? How 
long does it require to revolve around the sun ? How long on its 



THE EARTH. 37 

Europe in the sixteenth century and the discoveries of 
Copernicus, considered tlic earth as the centre of the 
universe. They regarded it as the great masterpiece of 
creation, around which the sun and moon, planets and 
starry heavens, revolved. The Egyptian astronomer did 
this, as well as the Grecian, the Roman, the Persian, and 
the Arabian. All imagined themselves the distinguished 
inhabitants of the great central orb of the universe. 

4. But this intellectual darkness was not to hang for- 
ever over the minds of men and enshroud the world. 
The principle of the law of gravitation was applied by 
Sir Isaac Newton to the heavenly bodies, whereby the 
relative weights of the sun and planets could be deter- 
mined; elements which enable us to prove that the earth 
has an annual motion. As we had occasion to notice, 
two bodies gravitate or are attracted towards each other 
with forces respectively equal to the amount of matter 
each contains and the distance that they are apart. This 
fact is illustrated by Fig. 5. 

5. If a heavy body is fastened on one end of a rod, 
and a light body on the other end, and this rod, at the 
point at which the two bodies balance each other, be 
supported by a fulcrum, or prop, that point on which 
it rests will be the centre of gravity of these two bodies. 
Now, if these two bodies are caused to revolve when 
connected, they will revolve around their common centre 
of gravity, and the lighter body will describe the larger 
circle, and the heavier body will describe the smaller one. 

6. By applying this principle to the sun and earth, 
it may be made plain that the earth has an annual motion 
around him. His weight is ascertained to be three 
hundred and fifty-five thousand times greater than that 



QuESTioxs. — 3. Ho^y did almost every system of astronomy view 
the earth before tlie discoveries of Copernicus? 4. Who applied 
the principle of gravitation to the heavenly bodies ? 5. If a large 
body and a small one were fastened on either end of a rod, the point 
at which they would balance would be called what ? If made to 
revolve, what would they revolve around? AVhich body would 
describe the smaller circle ? Which the larger ? 

4 



38 MOTION OF THE EARTH. 

of the earth, and, consequently, his attractive force would 
be as three hundred and fifty-five thousand is to one. 
This being the case, the centre of gravity of these two 
bodies must be very near the centre of the sun; and, as 
it is apparent that one or the other does revolve in an 
extended orbit, it is evident that it is not the sun, but 
the earth, that has the annual motion. 



SECTION XL 

failg STotioit of tlje €artlj. 

1. It rotates once on its axis in twenty-four hours, 
or the sun and starry heavens pass clear round it in the 
same length of time. The apparent passage of the sun 
from east to west, and the vicissitudes of day and night, 
declare that one or the other of these statements is true. 

2. If it is the sun that passes round in twenty-four 
hours, he must fly with inconceivable velocity ; for, by 
knowing his distance from the earth, and doubling it, 
and multiplying by three, we will have the measure of 
his orbit. By dividing the number of miles in this 
orbit by twenty-four, as there are twenty-four hours in 
a day, we will have his rate of motion, which would be 
over twenty-four millions of miles per hour. But this 
is not all. The constellations appear to pass clear round 
the heavens in twenty-four hours. 

3. The nearest heavenly body outside of our planetary 
system cannot be less than twenty millions of millions 
of miles distant from the sun; and how much farther 
away must some of them be, when it takes their light 

Questions. — 6. How much heavier is the sun than the earth? 
Where, then, is the centre of gravity of the sun and the earth ? 
Which must revolve around the other ? 

Sec. XI. — 1. What evidence is there that the earth revolves on 
its axis in twenty-four hours, or that the starry heavens revolve 
around the earth in the same period ? If the sun revolves around 
the earth, what must be his velocity per hour ? Must the constel- 
lations also revolve around the earth, if the sun does ? 



MOTION OF THE EARTH. 39 

thoiisaiids of years to reach iis, travellin(>: at the rate of 
twelve millions of miles per minute! The star named 
61 Cygni, which might for comparative distance be con- 
sidered as it were a sentinel on the outposts of the solar 
system, to make a revolution in the allotted time would 
have to travel with the inconceivable velocity of one 
billion four hundred millions of miles per second. 

4. Now, if the same process of calculation is applied 
to some of those bodies that are farthest removed from 
us, what must be their velocity! Light itself would 
fail — infinitely fail — to perform the task in making the 
circuit, let alone those innumerable self-luminous worlds, 
many of which are at such enormous distance from us 
that the human mind can have no conception of it. 

5. Another difficulty also besets the theory of the 
whole universe revolving around the earth, and not the 
earth revolving on its axis, in twenty-four hours. Every 
star visible to us, even with the use of the telescope, 
would have to make its circuit clear round the heavens 
in precisely the same length of time, whether compara- 
tively near or sunk in the abysmal depths of space. 

6. To imao'ine this would be to imao-ine what is not 

... 
only improbable, but utterly impossible according to the 

present constitution of nature, where the inconceivable ve- 
locities of so many thousands — yea, millions — of workls 
would be involved in one universal revolution. Hence 
we arrive at this conclusion, which is iu accordance with 
the Copernican theory of planetary motion, — that our 
earth is a planet, with a yearly and daily motion, and 
holds, at least to us, a conspicuous place, as the moon her 
satellite does in. the economy of the solar system. 



Questions. — 3. What is the distance to the nearest fixed star ? 
What would be the velocity per second of 61 Cygni if it revolves 
around the earth? 4, 5. Would the most distant stars have to 
make a complete circuit around the heavens in twenty -four hours 
if the earth did not revolve on its axis ? Would it be possible for 
them to do it as now constituted ? Could light do it, travelling at 
its usual rate ? What is its velocity per minute or second ? 6. What 
conclusion do we arrive at in relation to the motions of the earth? 



40 THE MOON. 

SECTION XII. 

1. The Moon, though apparently the largest, is act- 
ually the smallest, heavenly body visible to the naked eye, 
being only a little over two thousand miles in diameter. 
And she is not quite half as dense as the earth. She is 
fifty times less than the earth, and the length of her year 
is about twenty-nine of our days. ]\Iany peculiarities 
belong to her wdiich are not connected with the sun or 
planets, and phenomena are manifested by her which 
are nowhere else discoverable in the universe. 

2. She has three motions, — one around the earth, one 
on her axis, afld one as an attendant of the earth around 
the sun. She rotates once on her axis, which is inclined 
about 1° to the plane of her orbit, while she makes a revo- 
lution round the earth, keeping, of course, the same side 
always turned towards us. This- being the case, each of 
her days and nights is nearly one-half of a month in length. 

3. Though it is in a measure true that only the half of 
the moon next to us is visible, yet it is not strictly so. 
Her north pole being at one time a little inclined to- 
wards the earth, we can see beyond it; and at another 
time her south pole being inclined in the same way, we 
have a more extended view in that direction. 

4. In like manner is it in regard to her eastern and 
western sides or limbs. In consequence of the ellipti- 
city of her orbit, and consequent inequality of her angu- 
lar velocity, she reveals and conceals alternately new 
territory, as it were, at the ends of her apparent equato- 
rial diameter. 

5. These variations are called her librations in lati- 



QuESTiONS.— 1. Why does the moon appear so large? \^ hat is 
her mean distance from the earth ? Is she as dense as the earth : 
What is the length of her rear? 2. How many motions has she, 
and what are they ? Does she always keep the same hemisphere 
turned towards us? What is the length of her days and nights / 

3. Are her north and south poles turned towards us alternately r 

4. Are her eastern and western limbs turned towards us alternately? 
6. What are these variations of position called ? 



PHASES OF THE MOON. 



41 



tilde and longitude, and are not necessarily connected 
with her phases, which take place both before and after 
her change and full. 



SECTION XIII. 

Ijjasfs of t\^z gloou, 

1. If the moon is be- 
tween us and the sun in 
the west, she is at her 
change, and is invisible. 
When she moves east- 
ward ninety degrees, we 
can see one-fourth of her 
surface enlightened ; and 
if she moves eastward 
ninety degrees farther, 
we can see the half of her 
whole surface enlighten- 
ed. By continuing this 
revolution around the 
earth, she manifests less 
of her enlightened sur- 
face, till she becomes in- 
visible again in the west. 

2. Owing to her con- 
stantly shifting her posi- 
tion in relation to the 
earth, her face next to 
us is more and more en- 
lightened by the sun 
from her change to her 
full; and owing to the 
same cause, the light of 
the sun is gradually with 
drawn from it, from her 




Pig. 11. 



Questions. — 1. WTiere is the moon in relation to the sun and 
earth when at her change? 



42 SOLAR AND LUNAR ECLIPSES. 

full to change, thereby manifesting at various times the 
crescent, the disk, and the gibbous form. 

3. She makes about twelve and one-third of these 
lunations, or revolutions around the earth, in one year, 
and, to accomplish this, must of course move in her 
orbit at times with greater velocity than the earth, as 
she has a much greater distance to travel. This fact 
may be readily understood if we consider that she re- 
volves around the earth, as the earth revolves around 
the sun. 



SECTION XIV. 

3alm anb ITtrnar Eclipses. 

1. Eclipses of both sun and moon have been inves- 
tigated with great care by philosophers of nearly every 
nation, sometimes for the purpose of operating on the 
fears of the ignorant, and at other times for the purpose 
of adding another leaf to the pages of science. The 
maximum and minimum number of solar and lunar 
eclipses in one year cannot be more than seven, nor less 
than two ; and when there are seven, five may be of the 
sun and two of the moon, or three may be of the sun 
and four of the moon ; but when there are only two in 
the year, both will be of the sun. 

2. To explain these phenomena, we may observe that 
the orbit of the earth is an ellipse, having two points 
called foci, and the sun is always in one of them, as may 

Questions. — How much, enlightened surface does she exhibit when 
about ninety degrees east of her change ? Where is she in relation 
to the earth and the sun when she is full ? 2. Does her enlightened 
surface increase from her change to her full? Does it diminish 
from her full to her change ? In these variations, what forms does 
she exhibit ? 3. How many revolutions does she make around the 
earth in one year ? Does she travel at times faster than the earth ? 
For what reason is this true ? 

Sec. XIV. — 1. For what purpose have eclipses been studied? 
What is the maximum number of eclipses that can occur in one 
year ? How many may be of the sun ? How many of the moon ? 
2. Is the orbit of the earth an ellipse ? What is an ellipse ? Is the 
moon's orbit elliptical ? 



SOLAR AND LUNAR ECLIPSES. 



43 



be seen by observing Fig. 12. The same thing is also 
true of the moon. In consequence of the ellipticity of 



A 



EARTH 



m Moorj 



MOON 



ii 



D 



MOON 



MOON 



Fig. 12. 



44 SOLAR AND LUNAR ECLIPSES. 

her orbit, and the eccentric^ position which the earth 
occupies in it, she is at one time nearer to the earth than 
at another. 

3. When the earth is moving in any point of its orbit, 
as at A, Fig. 12, and the moon directly between the 
earth and the sun, and at her greatest distance from 
the earth, there will be an annular^ eclipse of the sun; 
that is, the shade of the moon will fall a little short of 
the earth, and the moon herself will obscure the central 
portion of the sun at the middle of the eclipse. Again, 
if the earth is moving in any part of its orbit, as at b. 
Fig. 12, and the moon directly between the sun and the 
earth, and at its least distance from the earth, there 
will be a total eclipse of the sun. 

4. Partial eclipses of the sun are produced by the 
moon when she does not pass directly between the earth 
and the sun. If the sun, moon, and earth were not in 
a straight line, but the moon only a little to one side 
and in between the other two bodies, only a part of the 
sun's disk would be obscured, as she would not pass 
centrally over him. 

5. This change of the moon, to either side of the sun 
north or south, is in consequence of the plane of her 
orbit not being coincident with the plane of the earth's 
orbit, but at an angle with it of a little over five degrees. 
If the planes of the moon's and the earth's orbits were 
coincident, at every new moon the sun would be eclipsed, 
and at every full moon she would be eclipsed. 

^ Eccentric — out of the centre. 
2 Annular — in the form of a ring. 

Questions. — 3. Where must the moon be in relation to the sun 
and the earth when there is an annular eclipse of the sun ? What 
portion of the sun is obscured in an annular eclipse ? In an annular 
eclipse, is the moon at her greatest distance from the earth ? In a 
total eclipse of the sun, is the moon between the earth and the sun ? 
In a total eclipse, is she at her least distance from the earth? 4. 
What is a partial eclipse of the sun ? When do they occur? 5. At 
what angle is the plane of the moon's orbit with that of the earth ? 
If the planes of these orbits were coincident, when would the sun 
and moon be eclipsed? 



i 



LUNAR ECLIPSES. 45 



SECnON XV. 

ITunav (Etlijjscs. 

1. To explain the lunar eclipse, we have only to con- 
sider the moon and the earth to have exchanged their 
places in relation to their distances from the sun, as 
exhibited at letter c, Fig. 12. If the moon at her full 
is on the outside of the earth's orbit from the sun, and 
the earth directly between her and the sun, the shade 
of the earth will cover her whole disk, or eclipse her 
totally. But if the earth is not directly between the 
moon and the sun, only a part of the shade of the 
earth will obscure a part of the moon's disk, which 
produces a partial eclipse, as may be seen at letter D, 
Fig. 12. 

2. In solar eclipses the western edge or limb of the 
sun is obscured first, and in lunar eclipses the eastern 
edge or limb falls first within the shade of the earth. 
This is the invariable order of these phenomena, owing 
to the fact that the sun is relatively stationary, and the 
moon revolves eastward at times with greater velocity 
than the earth. 



SECTION XVI. 

ITiurar Influences. 

1. Many happy influences are ascribed to the moon, 
and many grave accusations are laid to her charge. 
Passing by the superstitious credulity of the ignorant 
on this subject, we can conceive of only three influences 

Questions. — 1. In lunar eclipses, what two bodies must exchange 
their places in relation to the sun ? AVhere must the earth be in 
relation to the moon, that she may be totally eclipsed ? Where must 
the earth be, that she may be partially eclipsed ? 2. Which limb or 
edge of the sun is obscured first in solar eclipses ? Which in lunar 
eclipses ? What reason can you assign for these facts ? 

Sec. XVI. — 1. What number of lunar agents affect terrestrial 
objects ? What are they ? 



46 LUNAR INFLUENCES. 

which the moon can have, or is known to exert, on ter- 
restrial objects. These influences are produced by her 
light, her heat, and her attractive force. 

2. That she reflects sunlight is evident; and that light, 
according to the degree of its intensity, has an influence 
on chemical elements, the growth of vegetables, and 
animal life, is clearly established. With sunlight, which 
is three hundred thousand times greater than that of the 
moon, some substances grow pale, while others grow 
black when exposed to its influences. 

3. These changes are effected more in consequence, no 
doubt, of the exceeding sensitiveness of certain objects 
to the presence of light, than to any inherent power 
which may reside in it to produce them. If it requires 
such intense light as sunlight to produce such compara- 
tively slight changes on terrestrial objects as that of 
color principally, how little indeed must be the influence 
of moonlight on the same objects, when it is three hun- 
dred thousand times less, and only reflected on them ! 

4. In like manner is it with the heat which she reflects. 
It is nothing when compared with the heat of the sun ; 
for it requires one of the most powerful lenses, and it to 
be placed under the most favorable circumstances, to 
detect that she reflects any heat at all: therefore it could 
not produce many natural wonders on the surface of the 
earth. 

5. But, since we have not discovered any miraculous 
power in either of these agents when reflected by the 
moon, it may be supposed to exist in her attractive force. 
To explain this influence, it matters not whether we say 

Questions. — 2. Has sunlight any effect on animal life and the 
growth of vegetables ? How much greater is sunlight than that of 
the moon ? What influence has it on vegetation ? 3. Is this change 
of color more from an inherent power in the light or sensitiveness 
of the objects to the presence of light? What is the influence of 
moonlight in comparison with that of the sun ? 4. Does the earth 
receive any heat from the moon ? How is it determined ? Can it 
produce much change in terrestrial objects? 5, 6. Does the moon 
attract the earth ? What eifect has the mutual attraction of two 
bodies on each other ? 



LUNAR INFLUENCES. 47 

that inert bodies are endowed witli an inherent power 
Avhicli causes them to tend towards each other, or that 
this tendency results from power exerted on them uni- 
formly by the great Lawgiver. Whichever view we 
adopt, the results are the same. 

6. The only visible or known effect which the force 
of gravity or attraction has on either animate or inani- 
mate matter, is to change its place. Bodies in conse- 
quence of this natural property endeavor to get together, 
and their efforts are mutual, and mutual in proportion 
to the amount of matter that each contains, and the dis- 
tance they are apart. And were it not for the initial 
force which each heavenly body received in time or 
when launched into space, all worlds would gather into 
one chaotic mass. 

7. This law is universal as creation, and prevails 
wherever matter exists. Were it not so, wild confusion 
would characterize the whole empire of the Infinite 
Architect. But, notwithstanding this force is every- 
where exerted, yet it is not the parent or cause of every 
thing. Though we could not live -without it, still there 
is a limit to its functions, both in the heavens and on the 
earth. 

8. True it is that the attractive influence of the moon 
disturbs the waters of the ocean, because there is little 
or no cohesive attraction between them : yet I have not 
seen her force vegetation through a rock. So also is it 
true that she to a far greater degree disturbs the upper 
regions of our atmosphere: still I have not seen her 
squeeze water out of the clouds by lying on her back 
and turning up her horns. 

9. These phenomena are not discovered by the slow, 
plodding student of nature, who labors assiduously all 
his life and then gains comparatively little, but only by 

Questions — 7. Is this law universal wherever matter exists? 
Are there limits to its functions? 8, Does the attraction of the 
moon disturb the waters of the ocean ? Are the upper regions of the 
atmosphere aftected by it ? Can it disturb solid substances on the 
earth ? 



48 LUNAR ATMOSPHERE. 

tliose who can see spirits or read the events of the future 
by the twinkling of a star or some other circumstance 
equally significant. 



SECTION XVII. 

1. The centres of figure and gravity of the moon do 
not coincide, or, in other words, her centre of gravity is 
at a greater distance from her surface next to us than 
from the surface of her opposite hemisphere. This dis- 
covery has been confirmed by science; and it may account 
for the absence of a visible atmosphere around her. 
Her hither hemisphere is elevated about thirty-three 
miles above the general level of her surface if she w^as 
a perfect sphere, and it has a less specific gravity than 
other parts of her body. 

2. The elevation of her hither surface appears to have 
been the work of time; and if she at her formation had 
an atmosphere all around her, and seas on her surface, 
they would seek by their own gravity her lowest parts, 
as the hemisphere next us w^as being elevated. They 
would naturally flow to her opposite side, leaving the 
side next to us destitute of those fluids which originally 
belonged to it. But, whether this increase of volume 
and change in the specific gravity of a part of the moon, 
and consequences flowing therefrom, took place in time, 
it matters not as far as the fact is concerned that neither 
air nor water has been discovered on her surflice. 

3. According to a fixed principle in optics, light 
travels in straight lines, unless diverted by reflection 



Questions. — 1. Do the centres of figure and gravity of the moon 
coincide? Which centre is at the greater distance from us? How 
much higher is her hither hemisphere than the general level of her sur- 
face? Is its specific gravity less than other parts of her body? 2. 
If air and water were on the moon, would they seek her lowest parts 
by their gravity? Has water or air been discovered on the moon? 



LUNAR ATMOSPHERE. 49 

or refraction ; and the denser the transparent medium 
through which it passes, the greater is its refractive 
power. This fact may be illustrated by Fig. 13. ^ Let 
the darkest color in the figure represent glass, which is 
a denser medium than water; the next above, water, 
which is a denser medium than air; and the lightest 
color, air, w^hich is a rarer medium than water; then, 
if a ray of light is brought in contact with the top one 
at a certain point, they will manifest, as the ray passes 
through them, their different powers of refraction. 








illliilill!lll!lil!lllllliiililiiliillllillliilllliiiillilllliM 

Pig. 13. 

4. If a ray of light from the star s enters the atmos- 
phere obliquely, it wdll turn it slightly downwards till 
it meets the water, which will change its direction down- 
wards a little more till it meets the glass, which will 
change it still more in the direction of a perpendicular. 
This is in accordance with what we have noticed above : 
the denser the transparent medium, the greater is its 
refractive powder. 

QuESTioxs. — 3, 4. Does liglit travel in straight lines, if not ob- 
structed? Is liglit ever reflected or refracted ? What kind of trans- 
parent bodies refract it most ? 

5 



50 LUNAR SURFACE. 

5. Let us now apply this principle to test the presence 
or absence of a lunar atmosphere. If the moon had an 
atmosphere, even a thousand times rarer than our own, 
it would refract or change the direction of the rays of 
light if they should pass through it; but no such influ- 
ence is perceptible on the rays of light coming from 
fixed stars as they pass by the edge of the moon to our 
eyes. They move in straight lines, as is represented in 
Fig. 14, thereby indicating that they do not pass through 
any transparent material medium outside of the earth^s 
atmosphere. 

6. Objects one hundred and twenty yards in diameter 
may be seen on the moon's surface by the aid of the 
telescope, amd, as her poles are nearly at right angles 
with the plane of her orbit, it must be constantly cold 
in her arctic regions, so much so that water would freeze 
and snows accumulate, which would be visible if she had 
an atmosphere and they were there. But they have 
never been discovered, neither clouds nor changes of any 
kind which would indicate the presence of human intel- 
ligence, neither animal nor vegetable life. 



SECTION XVIII. 

1. By referring to Fig. 14, you may obtain a pretty 
correct knowledge of the general outlines and most pro- 
minent features of the lunar surface. In the south and 
southeastern and southwestern parts it is exceedingly 
mountainous. In the north and northeastern and north- 

QuESTiONS. — 5. "Would atmosphere, a thousand times rarer than 
our own, refract light ? Is the light of fixed stars as it passes by 
the edge of the moon refracted ? What does this prove ? 6. How 
small a body may be seen on the moon with a large telescope ? If 
snow would fall on the surface of the moon, could it be seen ? Are 
there any indications there of animal or vegetable life ? 

Sec. XVIII.— 1. Is the lunar surface level ? Where is it most 
mountainous ? 



LUNAR SUEFACE. 51 



western parts it is not so hilly : still, it is very far from 
being a level plain. 

2. Where very high mountains are not observed, the 
country appears level, except when examined by a tele- 
scope of very high power, and then it exhibits hundreds 




Figi 14,— Telescopic View of tlie Moon, 

of conical hillocks all over its surface. These low places 
are at various elevations with the general level of the 
lunar surface, and are of various colors, which may result 
from the nature of the different kinds of rock or soil 
from which the solar light is reflected. 

Questions. — 2. What is observed on the more level places on the 
moon ? Are the low places on the moon at the same level ? Are 
they of the same color? 



52 LUNAE SURFACE. 

3. Sir William Herschel supjDosed them to be seas, 
but modern discovery has dissipated these conjectures 
and revealed their true nature and character. Some of 
these low lands appear to be entirely surrounded by high 
elevations, and others appear more like an archipelago, 
where large isolated mountains rise up promiscuously 
in their interior. These mountains are most frequently 
conical in form, and the great majority of them at their 
summits resemble the crater of a volcano. 

4. Nearly the whole of the southern hemisphere 
which is visible is covered with them, and they vary in 
their height, from the little hillock which is scarcely 
perceptible, to the rugged mountain of ten thousand feet 
of elevation. Often they appear to be crowded one on 
another, as if they had been formed at different periods 
by one bursting up through the base of another. 

5. Their craters or openings are sometimes of enor- 
mous magnitude, many of them being twenty miles in 
diameter, and, without the elevations of the mountains 
in which they are, they sink sometimes to the depth of 
seventeen thousand feet below the general level of the 
surrounding country. They are generally cup-shaped, 
and often on their inner surfaces small openings or 
craters open out into the main crater, and frequently a 
small mountain rises up from the lowest part a few thou- 
sand feet, with a crater or opening in its summit. 

6. From some of these craters radiate a great number 
of white rays or lines, which diverge and extend out- 
wards several hundred miles in every direction. They 
reflect light of the same color with the interior of the 



Questions. — 3. What surround some of these low places ? Do 
large mountains ever rise up in them ? What is the shape of these 
mountains ? What do they resemble at their summits ? 4. What 
is the elevation of some of these mountains ? Do some appear to 
have burst up through the base of others ? 5. Are their craters or 
openings sometimes very large ? How far do some of them sink 
below the general level of the lunar surface ? What is their gene- 
ral shape ? 6. What radiates from these craters ? What color is the 
light that these lines reflect ? 



LUNAR SURFACE. 53 

craters, and they Hold their way over valley and hill till 
they terminate in the distance. By their color and di- 
rections we may infer that the m-aterial which formed 
them issued from these ojDenings anterior to other up- 
heavals which have elevated parts of them from the 
positions which tliey originally occupied. 

7. In like manner also do we find a few lines about 
a thousand yards in width and a hundred miles in 
length, passing over some of the most level portions 
of her surface. They resemble great highways prepared 
for public convenience: still, no weary traveller has ever 
been discovered by their sides, neither have the glowing 
wheels of Dives ever been known to have marked their 
dust. Like the highest lunar peaks and deepest caverns, 
they appear never to have been visited by any sentient 
being or any thing else, except floods of insufferable light 
by day and impenetrable darkness by night. 

8. JSTo animated being is known to be there, to witness 
these mighty desolations or disturb the monotony of this 
universal solitude. No flowing stream is there, to irri- 
gate these barren wastes or stimulate the plant to germi- 
nate; neither is the bird of the morning there, to hymn 
its harmonious song. No forest is there, to create the 
welcome shade; no rivulet, to give variety; no flower, 
to emit its fragrance; no oasis, to awaken the imagina- 
tion; no verdant landscape, to enchant the eye; no 
human voice, to break the unutterable silence; but she 
appears to have lost her pristine beauty, and now to pre- 
sent a surface distorted probably by her own volcanic 
agencies and internal convulsions. 



Questions. — 7. What may be seen on the more level places of the 
moon? What do they resemble ? What visits the moon ? 8. Is any 
animal or vegetable life on the moon ? What agency elevated her 
hither hemisphere ? 

5* 



64 



MAB6. 



SECTION XIX. 



1. Mars is about one hundred and forty -five millions 
of miles distant from the sun, revolves on his axis in 
about twenty-four and one-half hours, is seven times less 

than the earth, 
and travels in his 
orbit at the rate 
of fifty-five thou- 
sand miles per 
hour, making one 
revolution round 
the sun in twenty- 
three months. 
His diameter is 
a little more than 
four thousand 
miles, and his 
density is nearly 
the same as that 
of the earth. 

2. He appears 
much larger and 
more brilliant to us at one time than at another. This 
is owing to the constant changes which he makes in his 
position as he travels in his orbit. When nearest to 
us, he is only fifty millions of miles distant, and when 
in the opposite part of his orbit from us, he is more than 
two hundred and forty millions of miles distant from 
the earth. 

3. His daily motion on his axis is determined by 
certain marks which are visible by the use of the tele- 




Fig. 15.— Telescopic View of Mars, 



Questions. — 1. What distance is Mars from the sun? In what 
time does he revolve on his axis? How much less than the earth ? 
At what rate does he travel in his orbit? How long to revolve 
around the sun ? What are his diameter and density ? 2. Why does 
he appear larger at one time than another ? 



THE ASTEROIDS. 55 

scope. They appear and disappear, and afterwards re- 
appear, alternately, indicating not only that he is con- 
stantly changing his snrface next to us, but also, by their 
periodic return, the time of rotation on his axis, which 
would be the length of his day. 

4. Like all the planets exterior to the earth, he does 
not present the lunar phases. Being outside of the earth 
from the sun, look at him whenever he is visible, and 
nearly the lialf of his whole surface will appear en- 
lightened. His surface is diversified with hill and dale, 
island and continent, land and water; and liis seasons are 
not dissimilar to our own, 

5. Having a dense atmosphere, as some suppose, or 
soil of a peculiar color, he presents a ruddy appearance, 
and his temperate regions have been seen to change with 
the line of his seasons from a dark color to a brilliant 
white, indicating that in medium latitudes there is the 
alternation of summer and winter, and at his poles his 
lofty peaks are covered with perpetual snow. 

6. He has no satellite or moon, but travels over the 
trackless fields of space solitary and alone, yielding im- 
plicit obedience to those laws of force and motion which 
are inseparable from his vocation and his nature. 



SECTION XX. 

^bc gisteroibs. 



1. Leaving Mars, and passing outward from the sun, 
Ave enter into the domain of other worlds of recent dis- 
covery. The asteroids are over eighty in number, and 
cannot be said to hold a very conspicuous place in the 
planetary system, as they are nearly all invisible to the 

Questions. — 3. How lias his daily motion been discovered ? 4. 
Why does he not present, like Venus, the lunar phases ? Is his sur- 
face diversiiied? 5. What color is he, and does his color change? 
What does this change of color indicate ? 6. Has he any moons ? 

Sec. XX. — 1. What are the asteroids? Are they visible to the 
naked eve? 



56 THE ASTEROIDS. 

naked eye. They are small planets; whether the frag- 
ments of a stupendous world shattered by internal con- 
vulsions or external violence^ or formed as other planets 
were, no one can tell. 

2. Four years and one-half is about the average time 
that they require to revolve around the sun, and no doubt 
they follow^ the analogy of the primary planets, and have 
a daily motion on their axis. Some of them are very 
small, being only a few hundred miles in diameter, and 
travel in orbits which are very elliptical and very much 
inclined north and south. Only a few of them present 
sensible disks when examined by the telescope even 
under the most favorable circumstances. 

3. Nearly all of them have a pale-ash color, except 
Ceres, which is of a reddish hue; and they are surrounded 
with atmospheres of great extent, some of which appear 
to consist of nebulous matter. Their average distance 
from the sun is about two hundred and sixty-one mil- 
lions of miles, and all of their orbits are nearly the 
same distance from the sun: consequently the law regu- 
lating interplanetary spaces, suggested first by Kepler, 
a German astronomer, no doubt led to their discovery. 

4. He saw that if the distance from the sun to Mer- 
cury was doubled, it would be nearly the distance 
from the sun to Venus, and that if it was multiplied 
by three, it would nearly give the distance from the 
sun to the earth, &c. But when he applied this rule to 
the distance between the sun and Jupiter, the space 
between Mars and Jupiter was too great to conform to^ 
this law. This apparent interruption led him to infer 
that there was an unknown planet in that interval, which 
is now known to be occupied by so many little worlds. 

5. Early in this century this idea was actively re- 

QuESTiONS. — 2. What is the average time of their annual periods ? 
What kind of orbits do they travel in ? Do many of them exhibit 
sensible disks ? 3. What is their average distance from the snn ? 
What led to their discovery? 4. What was this law? 5. Who 
attempted to test its truth or falsity? Were their anticipations 
realized ? 



THE ASTEROIDS. 



57 



vived, and the science of the Old World attempted to test 
its truth or falsity by exploring those ethereal regions 
where conjecture at least pointed to another planet.. This 
being done, their anticipations were realized, and new 
though infant members were found to belong to the solar 
family. 

6. These discoveries led to closer and more protracted 
investigations, and, by the aid of the telescope, during the 
last half-century more than eighty of these planetoids 
have been found, conforming to those laws which regu- 
late their elder brethren in rendering their homage to 
the sun. 

7. The first four asteroids — Ceres, Pallas, Juno, and 
Vesta — were discovered between January 1st, 1801, and 
March 29th, 1807; and all the rest were discovered be- 
tween December 8th, 1845, and September 20th, 1865. 



NAMES AND DISCOVERERS OF THE ASTEROIDS. 

Names. Discoverers. 

Euphrosyne. .Ferguson, Washington. 

Pomona Goldschmidt, Paris. 

Polyhymnia. .Chacornac, Paris. 

Circe " " 

Leucothea.... Luther, Bilk. 

Atalanta Goldschmidt, Paris. 

Fides Luther, Bilk. 

Leda Chacornac, Paris. 

Lgetitia " " 

Harmonia Goldschmidt, Paris. 

Daphne " " 

Isis Pogson, Oxford. 

Ariadne " " 

Nysa Goldschmidt, Paris. 

Eugenia " " 

Hestia Pogson, Oxford. 

Melete Goldschmidt, Paris. 

Aglaia Luther, Bilk. 

Doris Goldschmidt, Paris. 

Pales " " 

Virginia Ferguson, Washington. 

Nemausa Laurent, Nismes. 

Europa Goldschmidt, Paris. 

Calypso Luther, Bilk. 

Alexandra.. ..Goldschmidt, Paris. 

Pandora Searle, Albany, N.Y. 

Mnemosyne-Luther, Bilk. 

Concordia " " 

Olympia Chacornac, Paris. 

Echo Ferguson, Washington. 





Names. 


Discoverers. 




1. 


Ceres 


.Piazzi, Palermo. 


31 


2. 


Pallas 


.Gibers, Bremen. 


32 


3. 


Juno 


..Harding, " 


33 


4. 


Vesta 


.Olbers, 


34 


5. 


Astraea 


.Hencke, Driesen. 


35 


6. 


Hebe 




36 


7. 


Iris 


.Hind, London. 


37. 


8. 


Flora 


. " " 


38. 


9. 


Metis 


.Graham, Sligo. 


39. 


10 


Hv2"eia 


..De Gasparis Naples. 


■10 


11. 


Parthenope 




41. 


12. 


Victoria 


.Hind, London. 


42. 


18 






4S 


14. 


Irene 


.Hind, London. 


44. 


15. 


Eunoraia.... 


.De Gasparis, Naples. 


45. 


Ifi 


Psyche 




46 


17. 


Thetis 


.Luther, Bilk. 


47. 


18. 


Melpomene. 


..Hind, London. 


48. 


19. 


For tuna 


" " 


49. 


20. 


Massalia.... 


.De Gasparis, Naples. 


50. 


21. 


Lutetia 


.Goldschmidt, Paris. 


51. 


22. 


Calliope 


.Hind, London. 


52. 


23. 


Thalia 


. " " 


53. 


24. 


Themis 


.De Gasparis, Naples. 


54 


25. 


Phocea 


.Chacornac, Marseilles. 


55. 


26. 


Proserpine . 


.Luther, Bilk. 


56 


27. 


Euterpe 


.Hind, London. 


57 


28. 


Bellona 


.Luther, Bilk. 


58 


29. 


Amphitrite. 


.Marth, London. 


59 


30. 


Urania 


.Hind, London. 


60 



Questions. — 6. What did these discoveries lead to? 7. When 
were the first asteroids discovered? How many have been dis- 
covered ? 



68 



JUPITEE. 



Names. Discoverers. 

61. Danae Goldschmidt, Paris. 

62. Erato...., Forster, Berlin. 

63. Ausonia De Gasparis, Naples. 

64. Angelina Tempel, Marseilles. 

65. Maximiliana. " " 

66. Maia Tuttle, Cambridge. 

67. Asia Pogson, Madras. 

68. Hesperia Schiaparelli, Milan. 

69. Leto Luther, Bilk. 

70. Panopea Goldschmidt, Paris. 

71. Niobe. Luther, Bilk. 

72. Feronia Peters, Clinton, N.Y. 

73. Clytie Tuttle, Cambridge. 



Names. Discoverers. 

74. Galatea Tempel, Marseilles. 

75. Eurydice Peters, Clinton, N.Y. 

76. Freja D' Arrest, Copenhagen. 

77. Frigga Peters, Clinton, N.Y. 

78. Diana Luther, Bilk. 

79. Eurynome.... Watson, Ann Arbor, U.S. 

80. Sappho Pogson, Madras. 

81. Terpsichore... Tempel, Marseilles. 

82. Alcmene Luther, Bilk. 

83. Beatrix De Gasparis, Naples. 

84. Clio Luther, Bilk. 

85. lo Peters, Clinton, N.Y. 



SECTION XXI. 



1. Jupiter and his attendant moons are a great pla- 
netary system of themselves. They are at a greater 
distance from the sun/ manifest more wonderful move- 
ments, and exhibit more wonderful phenomena, than any 
of the heavenly bodies which have been noticed. This 
planet is about ninety thousand miles in diameter, is 
thirteen hundred times larger than the earth, and re- 
volves around the sun in nearly twelve years, is four 
hundred and ninety-five millions of miles distant from 
him, and rotates once on his axis in nearly ten hours. 

2. His density is four times less than that of the 
earth ; and he may be regarded as having no seasons, in 
consequence of his axis being nearly perpendicular to 
the plane of his orbit. Perpetual summer reigns at his 
equator, and the temperature decreases in the direction 
of his poles, till cold more intense than was ever expe- 
rienced by man holds its unrelenting sway. 

3. Dark-colored belts cross his disk nearly parallel 
to his equator, and are supposed to be, by some astrono- 
mers, portions of the planet itself, brought into view by 
the removal of clouds and mists that exist in his atmos- 



QuESTiONS. — 1. How far is Jupiter from the sun? How much 
larger than the earth ? What is the length of his annual period ? 
What of his daily ? 2. What is his density ? Has he a change of 
seasons ? Why ? 



JUPITER. 



59 



phere, through which openings are made by currents 
circulating around him. Others suppose them to be 
atmospheric currents, in which is suspended cloudy 
matter, which, like our trade-winds or other currents, 
are constantly revolving around him. This conjecture 
has been in a measure confirmed by the recent disco- 
veries which have been made in regard to the currents 
in our own atmosphere. 




Fig. 16.— Telescopic View of Jupiter, 

4. Aeronauts^ inform us that only a few miles from 



^ Aeronauts — persons that ascend in balloons. 

Questions. — 3. What appear on his disk ? What are they sup- 
posed to be ? 



60 Jupiter's moons. 

the surface of the earth there are atmospheric currents 
on either side of the equator, and nearly parallel to it, 
blowing constantly in one direction. If these currents 
were filled with dense cloudy matter, it is probable, if 
we were on another planet with the facilities for obser- 
vation that we have here, w^e might witness objects 
similar to those on Jupiter. But, whatever these may 
be, they are liable to slight fluctuations, which have 
been highly useful in obtaining a knowledge of the 
physical character of the planet, which we infer to be 
very similar to that of the earth. 



SECTION XXII. 

1 . Four moons attend this planet, and revolve around 
him in different periods. (See Fig. 3.) The first, or one 
nearest to Jupiter, is two hundred and seventy-eight 
thousand five hundred miles distant from him, is twenty- 
four hundred and forty miles in diameter, and revolves 
around him in a little over forty-two hours. 

2. The second moon in point of distance from the 
planet is four hundred and forty-three thousand miles 
distant from him, is twenty-one hundred and ninety in 
diameter, and makes a revolution around him in about 
three days and one-half. 

3. The third moon is seven hundred and seven thou- 
sand miles distant from its primary, is thirty-five hun- 
dred and eighty miles in diameter, and revolves around 
Jupiter in a little over seven days. 

Questions. — 4. What do aeronauts say concerning atmospheric 
currents ? Are the belts of Jupiter changeable ? 

Sec. XXII. — 1. How many moons has Jupiter ? What distance 
is the first one from Jupiter ? What is its diameter ? What is the 
length of its period ? 2. What is the distance of the second moon 
from Jupiter? What is its diameter? What is the length of its 
period ? 3. What is the distance of the third moon from Jupiter ? 
What is its diameter ? What is its period ? 



I 



jupitek's moons. 61 

4. The fourth moon from the planet is one million 
two hundred and forty-three thousand five hundred 
miles distant from him, is thirty hundred and sixty miles 
in diameter, and makes a revolution around him in nearly 
seventeen days. 

5. These satellites were discovered by Galileo, in sixteen 
hundred and ten, and were the first-fruits of the inven- 
tion of the telescope. They, like all the moons of all 
the other planets that have them, make only one revo- 
lution on their axes, while they revolve once around 
their primary. Their orbits, unlike the orbits of all the 
planets and satellites of the solar system, except the orbits 
of the moons of Uranus, are nearly circular, and the 
sum of all the light which they reflect is little in 
comparison with ^yhat is reflected on us in a moonlight 
night. 

6. Sunlight decreasing in intensity in an inverse ratio 
as it recedes from him, as we had occasion to illustrate, 
would be twenty-seven times less on their surfaces than 
on the surface of the earth. Three of Jupiter's moons 
are eclipsed at every revolution around their primary, 
and very frequently the fourth. This takes place owing 
to the fact that they revolve nearly in the plane of their 
primary's orbit. 

7. From these eclipses of the satellites of Jupiter 
the velocity of light has been discovered, and also by 
them terrestrial longitude may be determined. If the 
earth is between Jupiter and the sun, he would be one 
hundred and ninety millions of miles nearer to the 
earth than if the sun was between Jupiter and the earth, 
the difference of distance being the diameter of the 
earth's orbit. If Jupiter is either in opposition or con- 
junction, that is, at his least or greatest distance from 

QuESTioxs. — 4. AVhat is the distance of the fourth moon from 
Jupiter ? What is its diameter ? What is the length of its period ? 
5. Who discovered these satellites? What is the shape of their 
orbits ? 6. How many of Jupiter's moons are eclipsed at every re- 
volution? How much farther is Jupiier from the earth at one time 
than at another ? 



62 SATURN AND HIS RINGS. 

the earth, the exact time when any of his moons will 
pass behind him can be determined. 

8. It has been discovered that at these two points the 
apparent and computed times of obscuration do not agree 
by about sixteen minutes, indicating thereby that it re- 
quires light that time to travel one hundred and ninety 
millions of miles, or the distance that it is across the 
earth's orbit. By computing this velocity, we find it 
to be one hundred and ninety-two thousand miles per 
second, or twelve millions of miles per minute. 



SECTION XXIII. 

^atum anb I^is flings. 

1. We will now take into consideration Saturn and 
his rings, together with his cortege of satellites, which 
constantly surround him. He is about eighty thousand 
miles in diameter, is ten hundred times larger than the 
earth, and revolves around the sun in nearly thirty years, 
is nine hundred and nine millions of miles distant from 
him, and rotates once on his axis in a little over ten 
hours. His density is about that of cork-wood, or one- 
seventh that of the earth, and he has seasons similar to 
our own. 

2. This planet presents a pale-yellowish color, and, 
when examined by the telescope, belts are observed cross- 
ing his disk. From the fact that some of his satellites 
linger from fifteen to twenty minutes on his margin when 
passing behind him and emerging from their conceal- 
ment, it is inferred that he has a very dense atmosphere, 
this delay being accounted for by the refractive influence 

Questions. — 8. Has the velocity of light been determined by the 
eclipses of Jupiter's moons ? In what way ? 

Sec. XXIIi. — 1. What is the diameter of Saturn ? How much 
larger than the earth ? What is the length of his annual period ? 
What is his distance from the sun ? How long to rotate on his axis ? 
What is his comparative density? 2. What is his color and appear- 



SATURN AND PUS RINGS. 



63 



of a transparent medium. Admitting the law that re- 
gulates the diminution of light, the sunlight which he 
receives is ninet y times less than that which the earth 
receives. I 

3. Five rings, 
which may be 
naturally divi- 
ded into two 
grand divisions, 
surround him, 
and revolve with 
him in the plane 
of his equator, 1 
in the same 
length of time 
that he revolves 
on his axis. 
The two inner 
rings are of a 
darker color 
than the exte- 
rior ones, and 
the distance to 
the first from 
the body of the 
planet is about 
ten thousand 
miles, and the 
distance be- 
tween the darker 
rings is about 
fifteen hundred 
miles. The se- 
cond dark ring 
from the planet 
is about five ^^S- 17. Telescopic View of Satum. 




Questions. — 3. How many rings has he? How may they be 
divided? Can you give their dimensions and distances apart? 



64 SATURN AND HIS RINGS. 

thousand miles in width, and the distance between it 
and the first bright ring is about two thousand miles. 
The width of the inner bright ring is about twenty 
thousand miles, and the middle one ten thousand, and 
the outer five thousand miles. The space between these 
bright rings is about two thousand miles, and the thick- 
ness of each is about one hundred miles. 

4. These wonderful appendages have been studied 
with great care ever since they were first discovered, and 
still comparatively little is known concerning them. 
Sometimes they appear to break in pieces and afterwards 
reunite, as if they were composed of fluids similar to 
water. At another time they appear to change their 
positions, and expand and contract alternately, as if 
acted upon by some periodic agency. Recently it has 
been inferred that they are gradually closing in upon 
the planet, and that ere long they will deluge its surface; 
but this is not yet fully confirmed. 

5. They may ere long be acted upon by some com- 
pensating influence, by which they may be restored to 
their proper equilibrium and perpetuate their existence. 
It is known that mechanical forces are operating con- 
stantly on heavenly bodies ; and it may be that they will 
not only forever sustain, but have actually produced, 
these anomalous appendages. 

6. The form of a body often results from the forces 
which operate on it. If the earth was in the shape of 
a cube, and destitute of a daily motion, it would not 
remain so. Its outer parts would sink towards its 
centre, and other parts would be urged outwards, till 
equilibrium of gravity in its various parts would be 
restored. 

7. So, also, if a sphere is made to rotate rapidly on its 
axis, its axis will grow shorter, and its length will always 
be in proportion to the density of the body and the ve- 

QuESTiONS. — 4. Do these rings appear to change in form? What 
inference concerning them ? 5. Are mechanical laws operating on 
the heavenly bodies ? 6. If the earth was a cube, would it remain 
so? Why? 



I' 



SATURN AXD HIS lUXGS. 65 

locity of rotation. This, no douV)t, has been the case 
with the planets in assuniing the shape which they have. 
By revolving on their axes they have swollen ont at 
their equators and flattened at their poles, and they have 
swollen and flattened in proportion to their densities 
and velocities. 

8. The lighter the body and the greater the velocity, 
the greater would be the tendency to throw off from its 
surface. Xow, as Saturn is known to be composed of 
light material, and to rotate ra])idly on his axis, the 
lightest particles may have been thrown otf and assumed 
the annular form. 

9. In this theory we assume what all no doubt will 
be ready to admit, that matter was created out of nothing 
by the fiat of Omnipotence, and that it is always accom- 
panied by laws by which it is regulated and controlled. 
The fact is patent to all that natural causes are con- 
stantly producing natural results; and may it not have 
been so in the production of Saturn's rings'? 

10. Reason has not taught us the contrary, neither 
has revelation ; for we are nowhere informed whether 
worlds were spoken into existence or formed by physi- 
cal laws, which laws are of necessity the ordinances of 
the Great Architect of all things and Master-Builder 
of the Universe. But without a sufficient number of 
self-evident truths, from which we might deduce an 
irresistible argument, it is wisdom 'not to speculate too 
much on subjects suggested by discoveries made in the 
boundless field of space. 

11. It is not for us to dictate, but to examine, to re- 
flect, to reason, and confirm, if we can, what appears to 



Questions. — 7. If a spherical body rotates rapidly on its axis, is 
there any tendency to a change of form ? In what direction is this 
tendency? 8. May the rotation of Saturn on his axis account for 
his form and his rings? 9. Are natural causes constantly pro- 
ducing natural results ? 10. Are we anywhere informed whether 
worlds were spaken into existence by the Creator or the result of 
natural laws? 11. "VMiat is the office of reason in the inyestigation 
of the works of nature ? 



Q6 

be the teachings of the book of nature, on which is 
stamped the impress of divinity, and from which are 
reflected the natural perfections of an Infinite Creator. 



SECTION XXIV. 

datum's Poons. 

1. Saturn is attended by eight moons, and they have 
received the names of Mimas, Enceladus, Tethys, Dione, 
Rhea, Titan, Hyperion, and Japetus. (See Fig. 3.) 
These bodies are only visible by the aid of powerful tele- 
scopes, and seven of them nearest to their primary 
revolve around the planet in orbits whose planes are 
nearly coincident with that of the rings. Their diame- 
ters vary from five hundred miles to thirty-three hundred, 
and their periodic times from twenty-tw^o and one-half 
hours to seventy-nine and one-half days. 

2. llimas. — This satellite was discovered by Sir Wil- 
liam Herschel in seventeen hundred and eighty-nine, 
and is seventy-eight thousand miles distant from Saturn, 
and revolves around him in about twenty-two and one- 
half hours. 

3. Enceladus. — Sir William Herschel discovered this 
satellite also in seventeen hundred and eighty-nine. It 
is one hundred and twelve thousand miles distant from 
the planet, revolves around it in about one day and eight 
hours, and appears like a star of the fifteenth magnitude. 

4. Tethys, the third moon from Saturn, was disco- 
vered by Cassini in sixteen hundred and eighty-four, 
and it resembles a star of the thirteenth magnitude. It 
is one hundred and forty-eight thousand miles distant 
from the planet, and makes a revolution around its 
primary in nearly two days. 

Questions. — 1. How many moons has Saturn ? Are they visible 
to the naked eye ? 2. What distance is Mimas from Saturn ? What 
is the length of its period ? 3, 4, 5, 6, 7, 8, 9. What is said of all the 
rest of Saturn's satellites? 



II 



SATUEN^S ^rOONS. 67 

5. Dione. — This satellite was discovered by Cassini 
also in sixteen hundred and eighty-four, and it resem- 
bles a star of the twelfth magnitude. It revolves around 
its primary in nearly three days, and is distant from it 
two hundred and forty thousand miles. 

6. Bhea. — Cassini discovered this moon also in six- 
teen hundred and seventy-t^YO. It appears to vary in 
magnitude. At one time it resembles a star of the ninth 
magnitude, at another the tenth, at another the eleventh, 
and at another time the twelfth. It is two hundred and 
ninety-six thousand miles distant from Saturn, and re- 
volves around him in about four days and one-half 

7. Titan, the sixth and largest satellite of Saturn, 
was discovered by Huyghens in sixteen hundred and 
fifty-five. It is like a star of the eighth magnitude, 
is seven hundred and thirty-eight thousand miles distant 
from the planet, and revolves around it in nearly sixteen 
days. 

8. Hyperion. — This satellite was discovered by Pro- 
fessor Bond, of America, and Lassel, of England, about 
the same time, in eighteen hundred and forty-eight. It 
resembles a star of the seventeenth magnitude, is nine 
hundred thousand miles distant from its primary, and 
revolves around it in about twenty-one days. 

9. Japetus, the outermost satellite of Saturn, was 
discovered by Cassini in sixteen hundred and seventy- 
one. It is two millions two hundred and twenty-eight 
thousand miles distant from the body of the planet, and 
revolves around it in a little over seventy-nine days. 
The plane of its orbit is inclined to that of the rings 
about ten degrees, and the periodic changes in its light 
indicate that it makes one revolution on its axis while 
it is making one revolution around its primary. 

10. All of these satellites revolve around Saturn exte- 
rior to his rings, presenting with them to the Saturnians, 
if there are any, one of the most wonderful displays of 

Questions. — 10. Where in relation to Saturn's rings do all his 
satellites revolve ? If Saturn is inhabited, what natural exhibition 
will the inhabitants witness everv dav ? 



68 



URANUS, HEESCHEL OB GEORGIUM SIDUS. 



physical grandeur that is anywhere witnessed in the 
natural universe. Imagine one moon rising and another 
setting, one entering into an eclipse and another emerg- 
ing from it, one appearing as a crescent and another 
with a gibbous phase, and sometimes all of them full- 
orbed. Imagine also the rings of the primary at one 
time eclipsing the stars and illuminating the sky with 
their, splendor, and at another casting a deep shade over 
certain regions of the planet and unveiling the wonders 
of the starry firmament. Imagine, I say, such scenes 
as these, and is it possible not to regard them worthy of 
a Divine Being to unfold and even angels to contemplate? 



SECTION XXV. 

'Uranus, Jcrstljel, or (ieorgium ^ibus. 

1. This planet has three names, and was discovered 

by Sir William Her- 
schel in seventeen 
hundred and eighty- 
one, as if it were by 
accident, when en- 
gaged in a systematic 
examination of the 
heavens. He at first 
supposed it to be a 
comet, but subse- 
quent observations 
established its plan- 
etary nature. It is 
of a pale color, and 
exhibits no marks 
Pig. IS.-TJranus. whereby we may de- 

tect an atmosphere or daily motion ; but, reasoning from 
analogy, w^e may infer that it has both. 




Questions. — 1. Who discovered this planet? 
first suppose it to be ? 



What did he at 



SATELLITES OF URANUS. 69 

2. It is one billion eight hundred and twenty-eight 
millions of miles distant from the sun, and revolves 
around him in a little over eighty-four years. Its dia- 
meter is thirty-three thousand miles, and its density is 
the same as that of Jupiter, or one-fourth that of the 
earth. Its inhabitants, if it has any, receive three hun- 
dred and sixty times less sunlight than we do, unless 
there is some agency (as there may be) associated with 
the planet itself to increase its intensity. 



SECTION XXVI. 

Satellites of Ulrauus. 

1. Sir \\'illiam Herschel discovered six moons at- 
tending this planet; but owing to the fact that it requires 
powerful telescopes, used under the most favorable cir- 
cumstances, to see even some of them distinctly, little 
is known concerning them. (See Fig. 3.) 

2. Ariel. — This satellite revolves around its primary 
in two and one-half days. Umbrlel, the second moon 
in the order of distance from its primary, has a period 
of four and one-sixth days. Oberon, the third satel- 
lite, has a period of nearly eleven days. 

3. Titania, the fourth satellite in point of distance 
from its primary, has a period of thirteen and one-half 
days. The periods of the fifth and sixth moons around 
their primary have not been determined, on account of 
the great difficulty of seeing them even under the most 
favorable circumstances. 

4. The orbits of these satellites, like the orbits of the 

Questions. — 2. What is its distance from the sun ? What is the 
length of its annual period ? What is its diameter ? AVhat its re- 
lative density ? What is the relative amount of light that it receives ? 

Sec. XXVI. — 1. How many satellites has Uranus ? Is it diffi- 
cult to see them ? What is the length of the period of Ariel ? What 
of the period of Umbriel ? What of the period of Oberon ? 3. What 
of the period of Titania V What has prevented the periods of the 
tifth and sixth from being determined ? 



70 



NEPTUNE. 



satellites of Jupiter, are nearly circular, and, contrary to 
all the motions of all the planets and satellites of the 
solar system, they revolve from east to west, and in a 
plane nearly perpendicular to the plane of the orbit of 
their primary. 



SECTION xxvn. 



1. Since Uranus was discovered by Sir William 
Herschel, it did not at times conform to the orbital 
motion which was anticipated by those who determined 

the elements of 
its orbit and cal- 
culated where it 
should be at any 
given time. In 
consequence of 
these deviations, 
it was conj ectu red 
that there was 
some unknown 
body belonging 
to the solar sys- 
tem, which was 
by its attraction 
producing these 
irregularities in 
the movements 
Fig. 19 -Neptune. of Uranus. 

2. Mr. Adams of Liverpool, and Le Verrier of Paris, 
unknown to each other, attempted, by calculations 

Questions. — 4. What is the form of their orbits ? What is pecu- 
liar in relation to their motions? 

Sec. XXVII. — 1. Does Uranus ever leave his computed orbit? 
What was conjectured from his deviations? 2. Who attempted to 
account for these perturbations ? Were they engaged at this work 
at the same time ? 




SATELLITES OF NEPTUNE. 71 

founded on the deviations of Uranus from his com- 
puted orbit, to determine where tliis disturbing cause 
was situated. Both of these mathematicians, nearly at 
the same time, succeeded in pointing out in the heavens 
where it sliould be found. 

3. Le Verrier having completed his computation in 
eighteen hundred and forty-six, a little earlier than 
Adams, wrote to Dr. Galle of Berlin, directing him 
Avhere to examine the heavens with his telescope for 
this conjectured body. The Dr., conforming to the 
request at the earliest convenience, employed his instru- 
ment in exploring that region of the heavens to which 
he had been directed, and, according to the prediction 
of Le Verrier, the planet Neptune was for the first time 
observed. 

4. This body is more than twenty-eight hundred 
millions of miles distant from the sun, and revolves 
around him in one hundred and sixty-four years. It 
is thirty-five thousand miles in diameter, and its density 
is the same as that of Saturn, or one-seventh that of the 
earth. He revolves nearly in the plane of the earth's 
orbit, and in all probability he has, like the other 
planets, a daily motion on his axis. Some observers 
supposed that they saw something that resembled a 
luminous ring around him; but this idea has not yet 
been fully confirmed. 



SECTION XXVIII. 

Satellites of ^cptnnc. 

1. Shortly after Neptune was discovered, Lassel of 
Liverpool discovered a satellite attending this planet 
and revolving around it in an orbit nearly circular. It 

QuESTioxs. — 3. AVho succeeded first ? "Who was the first observer 
of Keptune? 4. How far is this planet from the sun ? What is the 
length of his annual period ? What is his diameter ? What is his 
relative density? Are there anv indications of a rino- around him? 



72 LIGHT AND HEAT ON NEPTUNE. 

shines like a star of the fourteenth magnitude, is two 
hundred and thirty-two thousand miles distant from the 
planet, and completes a revolution around it in nearly 
six days. 

2. Owing to the variations in its brightness, it is in- 
ferred that it rotates once on its axis while it revolves 
once around its primary. Some other astronomers have 
affirmed that this planet has at least one other satellite; 
and recent discovery corroborates their previous con- 
victions by revealing the second satellite of Neptune. 

3. The general characteristics of Neptune and his 
satellites are, as far as can be determined, similar to 
those of the other planets and satellites, and their phy- 
sical constitutions are no doubt nearly the same. 



SECTION XXIX. 

WW aub feat on feptune. 

1. The light and heat which Neptune receives from 
the sun are nine hundred times less than what we receive, 
unless they are intensified by certain elements or influ- 
ences on his surface. That some substances have the 
power to intensify light and heat, and others power to 
diminish their influence, is a fact of which science is not 
ignorant. The atmosphere, together with the solid 
parts of the earth, even in the same latitude, modifies 
their effects to a very great degree. 

2. Low lands in the torrid zone are visited with 
scorching heat, whilst up the slopes of mountains may 
be seen evidences of a temperate climate, and on their 

Questions. — 1. Has Neptune any satellites? How many have 
been discovered ? How far is the first from the planet ? AVhat is 
the length of its period ? 2. Are there any indications of a second 
satellite? 3. What is the general character of Neptune and his 
moons ? 

Sec. XXIX. — 1. Hoav much less light and heat does Neptune 
receive from the sim than the earth ? Have different substances 
different effects on lijjht and heat? 



i 



LIGHT AND HEAT OX NEPTUNE. 73 

tops perpetual slieets of ice and snow. These different 
degrees of heat ahnost in the same vicinity cannot be 
attributed to any difference of distance that we are away 
from the source of heat, but to the nature and relation 
of the material elements with which the solar rays are 
brought in contact. 

3. The atmosphere gets rarer the higher we ascend, 
and thereby loses its power to refract, reflect, and retain 
heat, as is also the case with it in relation to the solar 
light. If we had no atmosphere capable of refracting 
and reflecting light, every thing here in the presence of 
the sun would be in one insufferable glare, and every 
thing not in his immediate presence would be sur- 
rounded by midnight darkness. 

4. If the sun did not look directly into your chamber, 
it would be a dungeon, and the shade which is so in- 
viting to the weary traveller would possess a gloom more 
palpable than was ever felt by man. No twilight would 
spread itself over the world to introduce the darker 
shades of night, neither would its mellow light intro- 
duce the brighter rays of morning. If we had no at- 
mosphere, which can in a measure modify other physical 
agents, in spite of the grateful changes of seasons, the 
earth would be a barren waste, chained in eternal ice 
and entirely destitute of life and beauty. 

5. Hence we observe the adaptation of one thing to 
another which tends to the comfort and happiness of 
our race. If we have susceptibilities of pleasure, there 
are objects corresponding whereby they may be grati- 
fied. If our appetites crave any thing, we have viands 
suited to the taste of the most fastidious; and if we de- 
sire to take a survey of natural scenery, and feast our 
eyes on the beauties of the landscape, or witness the 

QiTESTioxs. — 2. Why is it hotter on low lands than on high lands ? 
3. Does the atmosphere diminish in density as we ascend ? Does 
the atmosphere absorb heat and reflect and refract light? 4. Wliat 
would be the result if it did not ? 5. Is adaptation manifested in 
nature's works ? Might we infer fi'om this that other worlds are 
inhabited ? 



74 THE EARTH ONE OF A CLASS. 

plunging waves of the cataract, light is provided for 
our accommodation. If we are in need of any com- 
fort, nature has it in her great storehouse adapted to 
our wants. 

6. Hence, if the ministrations of nature to our wants 
are so abundant, may we not infer that there are millions 
of other intelligent beings on other planets, capable of 
enjoying the rich provision of comforts which, no doubt, 
crowd upon their surfaces? 



SECTION XXX. 

^\}z ^artlj om of a Class. 

1. The earth is one of a class; and that class is the 
primary planets, whose physical constitutions are much 
the same. If the earth is composed of solid material, 
and is warmed by the heat of the sun, so are all the 
other planets. If the earth has an atmosphere, and is 
visited with rain and hail and snow and frost, so are 
they. If she has rivers, oceans, seas, and continents, in 
like manner do they possess the same. If she runs an 
annual course around the sun and is constantly bathing 
herself in light, so do they. 

2. But it may be said that an inadequate quantity of 
light and heat to answer the purposes of life reaches 
some of these planets, and that they are buried in the 
abysmal depths of eternal night and winter. Follow 
the rays of heat and light which emanate from the sun 
to the farthest boundaries of our system, and still their 
influences are not entirely lost. A brilliancy equal to 
that of a hundred of our moons would be on Neptune, 



Questions. — 1. Is the earth one of a class? What is that class? 
Why do we infer that the earth belongs to a class ? 2. Does any 
sunlight or heat reach the outermost planet ? How much more sun- 
light is received on Neptune than the moonlight that we receive ? 
May he enjoy as much light and heat as planets that are nearer to 
the sun ? 



PLANETS INHABITED. 75 

and, by a slight change in his atmosphere from ours, he 
might experience as much heat as we enjoy. 

3. Science is familiar with the fact that the capacity 
of some substances on the earth to receive and retain 
caloric is greater than the capacity of others. Consider 
this to be the case with all the planets, and they may 
have different capacities for the same purpose, which 
may be conducive to their general well-being and the 
well-being of their inhabitants. The substance of ca- 
loric may be more abundant in bodies that are more 
remote than in those that are nearer to the sun, and it 
may be productive of sensible heat, even by a very slight 
influence of the solar rav. 



SECTION XXXI. 

glands Inljabitcb. 

1. On the hypothesis noticed above, which is corro- 
borated by a great variety of facts and experiments, 
there may be a sufficient amount of heat even on Nep- 
tune to satisfy all the demands of animal and vegetable 
life. And if there should be no substance there to 
operate on light in a similar manner, provision to supply 
this want may have been made in the animated beings 
themselves. 

2. The brilliancy of a day, or of light, depends to a 
very great degree upon the size of the pupil of the eye 
and the sensitiveness of the seat of vision. If the 
pupils of our eyes were comparatively small, and the 
pupils of the eyes of beings more remote from the 



Questions. — 3. How might this be the case ? Have some sub- 
stances a greater capacity for caloric than others? 

Sec. XXXI. — 1. May Neptune have enough of light and heat to 
satisfv' the demands of animal and vegetable life ? 2, On what does 
the brilliancy of light depend ? What would render the outermost 
planets, in relation to light and heat, suitable habitations for animated 
beings ? 



76 PLANETS INHABITED. 

sun than we are, were comparatively large, their day 
might be more brilliant to them than ours is to us. 
A slight diversity in the composition and nature of the 
various bodies of the solar system, and also in the 
construction of the organs of sight, would render every 
known planet as suitable and comfortable a habitation 
as we ourselves enjoy. 

3. In view of what is reasonable to suppose, and the 
striking analogy of the planets, not only in relation to 
their motions, but also in their physical constitutions, 
are we not led almost to the irresistible conclusion that 
there are not only worlds which declare the glory of their 
Maker, but immortal and intelligent beings not far 
distant, Avho adore the perfections and chant the highest 
symphonies of praise to the honor of the Most High. 

4. Why is there such provision made, and such an 
adaptation of things to the comfort and happiness of 
life, as we observe pervading these worlds, if there are 
no beings to enjoy them? Why such an exercise of 
almighty power in their production? Why such a 
demonstration of divine wisdom in their adaptation? 
Why such a glorious display of infinite goodness in their 
arrangement, if they were to be uninhabited solitudes 
and forever destitute of rational and intelligent beings, 
who may magnify the perfections of the Creator in the 
contemplation of his wonderful works and in the ful- 
filment of the great end of their existence? 



Questions. — 3. In view of the analogy of the primary planets 
in relation to their motions, physical constitutions, and other cha- 
racteristics, what may we infer? 4. In view of their creation, 
arrangement, and adaptation to the wants of intelligent beings, 
what may we infer ? 



4 



CONFIGURATION OF THE HEAVENLY BODIES. 77 



SECTION XXXII. 

(l?)fnrral Configmation of i\]t prabfnlg '§o'inzB. 

1. The genera] configuration of the earth's surface 
is splierical, or globular, which may be confirmed by the 
following considerations. 1st. When a shij^ is approach- 
ing land at a distance, the top of the mast comes first 
into view, and more and more of it becomes visible as 
she comes nearer, till the vessel itself is distinctly ob- 
served as she comes into port. 

2. By travelling constantly in one general direction, 
which has been done by navigators, they arrive at the 




Fig. 20 —Earth's Shadow, 



same place from whence they departed. Captain Cook 
and others have repeatedly demonstrated this fact by 
their voyages around the world. 

3. The rotundity of the earth is also apparent from 
the phenomena manifested by eclipses of the moon. 



QuESTioxs. — 1. Is the earth globular id form ? What is the evi- 
dence noticed in first verse ? What in second verse ? What in third 
verse ? 

7* 



78 CONFIGURATION OF THE HExlVENLY BODIES. 

When the earth intervenes between the sun and moon, 
she casts her shade on the moon^s disc, and it makes a 
circular figure, indicating thereby that the body which 
produced it is globular in form. (See Fig. 20.) 

4. The rotundity of the earth is more fully established 
by the mutual attraction existing between all the parts 
of it, and all the particles of matter that compose it. 
The tendency of every particle in a body is to sink 
towards its centre; and if it was of any other shape 
than spherical, the outer parts would crush inw^ards by 
their own weight or gravity, and other parts would be 
urged outwards, till equilibrium of gravity of all its 
parts would be arrived at ; and this could occur only 
when all of the parts are to a very great extent equi- 
distant from the centre. 

5. In like manner may we reason in relation to the 
spherical form of all the heavenly bodies. As the law 
of gravitation exerts its influence on matter wherever it 
exists, and as its influence is always the same in kind, 
similar results would of necessity flow from similar 
causes. The attractive force being always towards the 
centre in all bodies, the particles which constitute them 
would so arrange themselves as to render their exterior 
surface convex or spherical in form. 

6. Observation also fully confirms what we have 
already said in relation to the figures of the planets and 
other heavenly bodies. It matters not in what position 
they may be, or what relation they may sustain to each 
other, when viewed with the naked eye, or with the use 
of the best optical instruments, they appear uniform in 
their general configuration. 

7. The various phases of the moon clearly indicate 
her figure, and that she shines by reflection. As she 
moves eastward after her change in her orbit faster 

Questions. — What in fourth verse ? 5. From the efiects of what 
law may we infer that all heavenly bodies in general are spherical 
in form? 6. Do they appear round to the naked eye? Do they 
with the use of the telescope? 7. What peculiar evidence that the 
moon is round? What more do we learn from her phases? 



SPECIFIC FOKM OF THE EARTH. 79 

than the earth does in hers, her surface next to us day 
by day becomes more and more enlightened till she is 
full. These peculiar phases of light, which she mani- 
fests every successive night, as she and the earth change 
their relative positions, could not occur were it not that 
it is reflected by a body which is spherical in form. 

8. The same is also true of Mercury and Venus. 
They, in consequence of revolving around the sun in 
orbits which are nearer to him than the orbit in which 
the earth revolves, exhibit similar phases when they are 
examined by the telescope. By these i)hases they exhibit 
their forms, which are evidently analogous to that of all 
the planets and satellites, and also to that of the sun 
himself. 

9. He revolves on his axis in about twenty-five and 
one-half days, and in making this revolution exposes 
his Avhole surface to view. During this whole period 
no change of form is manifested, which of necessity 
would appear if he had not a surface whicli was uni- 
formly convex. In view of the foregoing evidence, and 
other considerations almost as conclusive, it may be 
inferred that the general configuration of all heavenly 
bodies is now round or globular, whatever may have 
been their condition in time, or when first brought into 
existence. 



SECTION XXXIII. 

^jjetifit J'orm of tlje €'arlb. 

1. Notwithstanding the earth's general form is 
globular, still it is not a perfect sphere. It is flattened at 
the poles, and swollen out at its equator, making the polar 

Questions. — 8. Do Mercury and Venus exlnbit different phases, 
like the moon ? What may we infer from them ? 9. Does the sun 
exhibit any change of form as he revolves on his axis? Why not? 

Sec. XXXIII. — 1. Is the earth a perfect sphere? Where is it 
flattened, and where is the excess of matter? How nmch shorter is 
its polar than its equatorial diameter? A globular body, if rotated 
Mpidly on its axis, has a tendency to assume what form ? 



80 SPECIFIC FORM OF THE EARTH. 

diameter about twenty-six miles shorter than the equa- 
toriah It assumed this peculiar form, probably, at the 
time of its formation, from the rapid motion which it 
has on its axis. By revolving a globular body on which 
water is placed at a certain rate of motion^ the water 
wnll leave the poles of the body and accumulate about 
the equator, changing its form into that of an oblate 
spheroid.^ This is now the form of the earth, as is evi- 
dent from the following considerations. 

2. If the earth was a perfect sphere, the pendulum of 
a clock which vibrates once every second at the equator 
would vibrate once every second at the poles. By ex- 
periment, its vibrations are found not to be uniform all 
over the earth, but they invariably increase as the clock 
is removed away from the equator. This increase of 
action in the clock is in consequence of its nearer ap- 
proach to the centre of the earth as it advances towards 
the poles. The attraction of gravitation always increases 
inversely as the square of the distance, and as tlie dis- 
tance decreases the attraction increases, which increases 
the vibration of the pendulum. 

3. For the same reason, bodies w^igh more near the 
poles than at the equator. The weight of a body is 
the measure of the earth's attraction for it, and the 
nearer that it is to its centre, so that it is on its surface, 
the greater will be its weight. It has been ascertained 
by actual experiment that bodies are not uniformly 
heavy all over the earth, but that they increase in weight 
as they are removed from the equator. This change of 
v>eight can only be accounted for on the principle that 
the earth's diameter is shorter in one direction than 

^ Oblate spheroid — is a solid figure slightly flattened on the opposite 
sides. 

Questions. — 2. Do the vibrations of thependnlnm of a clock afford 
any evidence that the earth is not a perfect sphere? In what way? 
Why docs it vibrate faster in the direction of the poles? 3. What 
is the weia:lit of a body ? Do bodies weigh as much at the equator 
as in the direction of the poles ? What does this difference in weight 
prove ? 



I 



PLANETS OBLATE SPHEROIDS. 81 

ill another, and that objects on its surface are not all 
equally distant from its centre. 

4. But these are not the only facts that may be 
adduced to prove the peculiar form of the earth. It 
is ninety degrees from the equator to the poles. All 
of these degrees are known to vary more or less in their 
length. They have been discovered to increase more 
and more in length as they are measured from the 
equator. If they were measured on a perfect sphere, 
they would be uniform in length; but the opposite is 
true, it matters not where the experiment is made. 



SECTION XXXIV. 

^t |Ianets Ablate ^p^eroibs. 

1. Oblateness^ is manifested by some of the planets 
that are comparatively near to us, as well as others that 
are known to revolve rapidly on their axes which are 
more remote. Mars exhibits a difference between his 
polar and equatorial diameters of twenty-five miles ; and 
if he revolved more rapidly on his axis than he does, 
no doubt it would be much more. 

2. Jupiter illustrates this idea by his oblateness. He 
is about two hundred and seventy thousand miles in 
circumference, and, in consequence of his making a rota- 
tion on his axis in about ten hours, the parts at his 
equator must revolve with great velocity. As a result 
of this, he is very much flattened at his poles. 

3. Each extremity of his polar axis is shortened at 
least three thousand miles ; and those of the polar axis 

^ Oblateness — fatness at the poles. 



QL'ESTioxs. — 4. What additional evidence that the earth la 
flattened at the poles? 

Sec. XXXIV.— 1. Is Mars flattened at his poles? How much? 
2. Is Jupiter flattened much or little at his poles? 3. How mucli 
shorter is his polar than his equatorial diameter? 



82 POSITION OF THE SOLAR SYSTEM. 

of Saturn are shortened still more, and very much more, 
considering his relative size, but no more when we con- 
sider his relative density. From these observations, 
made on some of the most prominent planets, under the 
most favorable circumstances, it is altogether probable 
that the same is true of all heavenly bodies that rotate 
rapidly on their axes. 

4. The. laws of force and motion will always produce 
similar results under similar conditions, whether they 
operate on the earth or in the heavens. Their power 
and influence are felt by all worlds, and unto them do 
they conform, not only in their general movements, but 
also in their individual and specific forms. Nature's 
laws always operate uniformly in shaping and con- 
trolling the material universe, and in satisfying the \Yill 
of Him who is the Author of them all. 



SECTION XXXV. 



Jositiou of t^e Solar Sgstem in relation to t\^z Slarrg 

1. As has been already noticed, the clusters or groups 
of stars that are visible are named constellations. All 
of these constellations, for the sake of convenience, are 
divided into three grand divisions, — the Northern, the 
Southern, and the Zodiac. As the first two divisions 
are not so intimately connected with our present subject, 
we will refer to them hereafter, and consider only that 
division which has a direct relation to it. (See Fig. 21.) 

2. The- zodiac is composed of twelve signs, and with 
each sign is associated a constellation. Each sign is 



Questions. — Is Saturn flattened at his poles more or less than 
Jupiter? Does oblateness appear to be common to all the planets? 
4. Of what natural laws is it the result? 

Sec. XXXV. — 1. Into how many grand divisions are the starry 
heavens divided ? What are they called ? 2. How many signs con- 
stitute the zodiac? How manv constellations? 



POSITION OF THE SOLAR SYSTEM. 



83 



thirty degrees in length, or east and west, and sixteen 
in width, or north and south ; each constellation is co- 
extensive with each sign, — that is, the one in area is 
equal to the other. These twelve constellations are so 




Fig, 21 —Zodiac, 

arranged as to form a great belt or zone of stars which 
extends clear around the heavens, east and west, and 
divides them into two equal parts, known as the northern 
and southern divisions. 

3. Hence these divisions are not separated by one 



QuESTio^rs. — What is the length and width of each sign? Is each 
constellation coextensive "with each sign ? AYhat is the form of the 
zodiac, and where is it situated in the heavens? "What divisions 
does it separate ? 3. To which of the three grand divisions is the 
solar svstem most intimatelv related? 



84 POSITION OF THE SOLAR SYSTEM. 

common circular line, but by the zodiacal division, which 
lies between them, and is sixteen degrees in width. To 
this latter division the solar system may be regarded as 
being more intimately related, not in consequence of its 
contiguity to any part of it, — for they are millions of 
millions of miles apart, — but in consequence of the 
astronomical connection of the one to the other. 

4. The plane ^ of the earth's orbit, if extended 
inwards from the earth towards the sun, would divide 
him into two eqnal parts, and if extended outwards 
into the heavens it would not only divide them into 
two equal parts, but the zodiac also, with eight de- 
grees on either side of it. Therefore, since it has 
been shown that the planes of the orbits of all the pri- 
mary planets are at very small angles with that of the 
earth, hence none of them, if extended into the sidereal" 
heavens, will pass beyond the limits of the zodiac, either 
north or south. 

5. In view of these conceded relations, the system to 
which our earth belongs may be regarded by us, if we 
are at or near the equator, as occupying a vertical posi- 
tion, similar to a wheel standing on its rim on a level 
plain, in an easterly and westerly direction. The centre 
of the wheel would represent the place of the sun, and 
the rim the orbit of Neptune, the intermediate space 
being occupied by the orbits of all the other planets, 
not many of which deviate far from the general plane 
of the zodiac. 

6. But if we were situated at or near either of the 



^ Plane of an orbit — an imaginary surface in wliich the orbit 
wholly lies. 

^ Sidereal — starry. 



Questions. — 4. How would the plane of the earth's orbit, if ex- 
tended into the starry heavens, divide the zodiac? Would the 
planes of the orbits of any of the primary planets, if extended into 
the starry heavens, extend north or south of the zodiac ? 5. What 
is the position of the solar system to the zodiac? How illustrated? 
6. If at either pole, what then is the position of the solar system, 
and where would the zodiac appear? 



MOTIONS OF THE PLANETS. 85 

poles, then the solai' system may be regarded as having 
a horizontal position, since the zodiac, instead of being 
always nearly overhead, would appear all around the 
horizon. Viewed as already stated, the sun Avould not 
at any time appear to rise high in the heavens as it does, 
neither would we have day and night every twenty-four 
liours, but he would appear to circulate around the 
earth not far above its surface for six months together, 
and below it the same length of time ; and each of the 
days and nights would be six months long. 

7. Hence it may be observed that the position of our 
system is not absolute, but relative, as it depends entirely 
upon the place that we occupy when we observe it, and 
the divisions of the heavens that surround it. As we 
change our place by going north or south, in like 
manner do they also appear to change ; and the one is 
in the same ratio as the other. 

8. But in going east or west there is no perceptible 
change in either, as we always travel in the same plane, 
and hold not only a common relation to the planetary 
system in general, but also to all the grand divisions of 
the heavens with which it is surrounded. 



SECTION XXXVI. 

Polious of the Jllaiuls Hrciinb tbe ^m\. 

1. The planets have two motions, when considered 
as parts of the solar system, the one around the sun, 
and the other on their axes. Each revolves around 
him from west to east, in periods varying from eighty- 
eight days to one hundred and sixty-four years. Mer- 



QuESTiONS. — 7. Is the position of the solar system absolute, or 
relative ? 8. Would these apparent changes take place by travellino: 
east or west ? 

Sec. XXXVI. — 1. How many motions have the planets? What 
are they ? In what direction do they travel around the sun ? Which 
has the shortest period ? Which the longest ? 

8 



86 MOTIONS OF THE PLANETS. 

cuiy, the planet nearest the sun, has the shortest period, 
and iS"ei)tune, which is at the greatest distance, has the 
longest, and those that are intermediate have periods 
varying according to their distances from the sun, and 
the speed at which they traveL 

2. The planet that has the least orbit moves with the 
greatest velocity, and as their orbits increase in size 
their velocities diminish, throughout the whole series 
that compose our system. These facts were discovered 
by observing the planets from time to time and noting 
their places in the heavens, when compared with the 
stars that are apparently fixed. 

3. At each successive period when observed, they 
were all found to have moved in one general direction ; 
and these observations l^eiug frequently repeated, the 
same results were always obtained. They pass appa- 
rently through one constellation after another in the 
zodiac, till they make a complete circuit around the 
heavens, always maintaining their easterly direction 
and relative motions. 

4. From this uniformity of direction of all the 
planets, and motions of each, their places in the heavens 
at any time can be accurately determined, their tran- 
sits and occultations^ can be safely predicted, and the 
true time when eclipses of both sun and moon shall 
take place can be definitely known. Such precision 
characterizes their movements that they are not one 
second of time too early or too late in their annual 
periods, but they constantly repeat them with perfect 
exactness. 

5. They are true to time not by chance, but by the 

^ Occultation of a planet — its concealment by the moon coming 
between it and us. 

QuESTioxs. — 2. Which travels with the greatest velocity ? Do 
they increase or diminish in velocity as their distances increase from 
the sun ? How is this fact known ? 3. How is it known that they 
all travel in an easterly direction ? 4. Can the place of each planet 
be determined at any time, and eclipses be predicted ? Can their 
transits and occultations be accuratelv foretold ? 



CENTRIPETAL AND CENTRIFUGAL FORCES. 87 

order of Him who can work by second causes as accu- 
rately and efficiently as he can directly by the power 
of his might. 

SECTION XXXVII. 

(Hentrt^ctal nub Centrifugal J^orces. 

1. If a ball is projected upwards in a vertical line, it 
will come to rest at a certain point in the air, and 
then it will descend in the same line till it comes in 
contact with the earth. Give it next an oblique motion, 
and it will not move in a straight line, but will com- 
mence and continue from the place where it received its 
first impulse, to describe a curved line till it comes to 
rest. 

2. In view^ of these facts, it is evident that more 
than one force operates on it, or it would have continued 
in a straight line in the direction which was given it by 
the projectile force. And it is further apparent that 
the force that brouo^ht it to rest was greater than the one 
that was at first imparted to it, as it entirely arrested its 
motion. This inequality of forces brought it to rest; 
for, were it possible to give a body such an impulse as 
to carry it clear round the earth to the same point 
exactly from which it left, it would continue to revolve 
perpetually, since the two forces, the one given to it and 
the earth's attraction which now operates on it, are equal. 

3. Under such conditions are the planets retained in 
their orbits. They are subject to these two laws, known 

Questions. — 5. Are the motions of the planets the result of 
chance, or according to the order and arrangement of the Creator 
and upholder of the universe ? 

Sec. XXXVII.— 1. What causes a ball or body to fall to the 
earth -when thrown up into the air? What kind of a line will a 
body describe if thrown in an oblique direction ? 2. How many 
forces operate on a body that is thrown into the air? What are 
they ? Which is the stronger force ? If an impulse could be given 
to a body sufficient to carry it round the earth to the point that it 
ie£t, would it continue to revolve perpetually ? 3. How are the 
planets retained in their orbits ? 



»<3 CENTRIPETAL AND CENTRIFUGAL FORCES. 

as the centrifugal^ and centripetaP forces. The centii- 
fugal force would always cause them to move in straight 
lines, if there was no impediment in the way, or other 
disturbing cause, to change their course. It is the initial 




Fig. 22. 

impulse which they received in the beginning when 
they came new from the Divine hand, or in time by the 
united force of natural laws. Whichever of these 
causes produced it, it matters not, inasmuch as each 

1 Centrifugal force — is the force wliicli is given to a body that causes 
it to move in a straight line. 

2 Centripetal force — is the force which draws a heavenly body 
towards its centre of motion. 



Questions. — What are the centrifugal and centripetal forces? 



CENTRIPETAL AND CENTRIFUGAL FORCES. 89 

planet has received it in a degree suitable to its position 
and the end for which it is designed. 

4. This centrifugal force is impressed on the planets 
near the sun to a greater degree than on those at a 
greater distance, and must necessarily be so to counter- 
act his attraction for them, which is the centripetal 
force. He draws them constantly towards himself with 
forces corresponding to their distances and their masses, 
and would sooner or later bring them to himself, if 
there was no other influence to counterbalance his attrac- 
tion for them. (See Fig. 22.) 

5. But, the centrifugal and the centripetal forces being 
equal, the planets are not permitted to depart from their 
orbits by flying off into space or falling upon his sur- 
face. These two forces are so arranged not only in rela- 
tion to the planets, but also to every scheme and system 
of heavenly bodies, as to counterbalance each other, and 
thereby perpetuate their harmony and existence. 

6. Motion is essential to the plan of the universe, and 
upon the equality of the centrifugal and the centripetal 
forces does it constantly depend. If the latter should 
fail or grow^ weak, physical anarchy would reign 
throughout the habitable empire of space ; and if the 
former should lose its influence, all worlds would dash 
together, and form the funeral pile of nature. The 
whole universe stands upon, them ; and to detract from 
either would be to unhinge all nature and turn it back 
to chaos. 



Questions. — 4. Which planets in relation to the sun have the 
greater centrifugal and centripetal forces ? Do they in relation to 
the planets counterbalance each other ? 5. If the centrifugal force 
was greatest, what would result ? If least, what would result in rela- 
tion to the planets ? 6. Is motion essential to the plan of the uni- 
verse ? What would be the result in nature if either should fail ? 

8* 



90 MOTION OF THE PLANETS. 

SECTION XXXVIII. 

Pottott of tl^t '§lmxth oiT lljnr '^u^. 

1. The daily motion of the planets, like their yearly 
motions, is from west to east, and is always the same. 
Whether it has resulted from the operation of natural 
laws in time upon the matter that composes them respect- 
ively, or is the effect of a Divine impulse as the planet- 
ary system was being constructed, no one can tell. 

2. By observing certain portions of their surfaces 
concealing and revealing themselves alternately, the 
diurnal motions of six of them are known ; and analogy 
would lead us to believe that they all rotate in the same 
direction on their axes. In this respect they may be 
divided into two classes, — those that have a faster and 
those that have a slower motion. The former class 
embraces Mercury, Venus, the Earth, and Mars, and 
the latter Jupiter and Saturn, the daily motions of all 
the rest being unknown. The four former planets 
rotate on their axes in nearly equal periods of twenty- 
four hours each, and the two latter in periods of about 
ten hours each. 

3. Now, as this daily motion causes a part of the sur- 
face of each to turn constantly into its own shade, and 
other parts at the same time to present themselves to 
the sun, light and darkness must occur alternately on 
each of them at each rotation. Consequently, daylight 
at the equators of the former would continue about 
twelve hours, and night would continue the same length 
of time ; and on the latter, at their equators, each day 
and night would be about five hours in length. 

Questions. — 1. In what direction do the planets rotate on their 
axes ? Is it known definitely how they received this motion ? 2. 
What evidence have we of it? Do they all rotate with the same 
velocity ? Which planets have the slow motion ? Which the fast ? 
3. Does this motion produce day and night on these bodies ? How ? 
What would be the length of one of Jupiter's days? What of one 
of his nights ? 



1 



WEIGHT OF OBJECTS. 91 

4. Leaving the equators of the planets, and going 
towards their poles, the days and nights vary in length, 
though not by any changes in their motions, but on 
account of their spherical form and the various incli- 
nations of their axes. As their poles decline from the 
sun, their days decrease and their nights increase in 
length ; and as their poles incline towards him, the order 
in the length of their days and nights is reversed. 
Corresponding with these changes in the length of day 
and night on the different planets is the change of their 
seasons, to which we will hereafter refer. 



SECTION XXXIX. 

iqbt of Objects on tbc §un mxb |.Uancts. 

1. It is a well-known fact that some objects on the 
earth weigh more than others. A cubic foot of water 
weighs more than the same volume of air. This differ- 
ence in weight results from a difference in the amount 
of matter that each of these bodies contains, one of 
them being more compact or dense than the other. If 
the earth was less dense than it is, both of them would 
w^eigh less than they do, as the weight of each of them 
is the measure of the earth's attraction for each; and if 
it was more dense than it is, each of them would weigh 
more. 

2. It is now apparent that there are two causes which 
may, under certain conditions, cause objects of the same 
bulk or volume to weigh more or less. One is a differ- 
ence in the amount of matter that may be in each of 



Questions. — 4. Does the length of day and night on the different 
planets vary more at the equator or at the poles ? 

Sec. XXXIX. — 1. Does a cubic foot of air weigh as much as a 
cubic foot of water? What causes the difference? If the earth 
was denser than it is, what effect .would it have u^on their weight? 
What, if rarer? 2. What two causes may change the weight of 
bodies ? 



92 WEIGHT OF OBJECTS. 

them, and the other is a diiference in the degree of 
attraction which the body has for them on which they 
may be found. If they belong to bodies that contain 
more matter than the earth, they will weigh more ; and 
if they belong to bodies that contain less matter than 
the earth, they will weigh less. 

3. ISTow, as the sun and planets are of different magni- 
tudes and different densities, they attract objects that are 
on their surfaces with different forces, which would cause 
even the same object, if it could be transported from 
one to the other, to vary in its weight. An object 
weighing a pound on the surface of the earth would 
weigh a little more than a pound on the planet Mercury. 
The material of which he is composed being nearly as 
compact as lead, his attraction for objects that may be 
on his surface is greater than the attraction of the earth 
is for objects on its surface. 

4. This is not so with the attraction of Venus and 
Mars for objects on their surfaces. Either of them 
being less than the earth, and having about the same 
density, objects on them would weigh more or less in 
proportion to their magnitudes. As Venus is only a 
little less than the earth, the weight of an object there 
would be only slightly reduced when compared with 
what it would be here. In the case of Mars it would 
be only about half as much on his surface as on the 
earth's, since he is so much less in size. 

5. On Jupiter and Saturn it would be the reverse, as 
either of them is much larger and contains more matter 
than the earth. If these bodies were dense according 
to their magnitudes, the difference in the weight of 
objects on the earth's surface and theirs Avould be 
greatly increased ; and, even as it is, a pound on the 
earth would be increased in weight if transport€d to 



Questions. — 3. Does the attraction of the sun and planets difler in 
degree? What causes this difference ? Will a body that weighs a 
pound on the earth weigh as much on Mercury? 4. More or less on 
Venus or Mars? 5. More or less on Jupiter or Saturn? 



earth's atmosppiere. 93 

either of them. On Jupiter it would be increased more 
than double , and, though Saturn is not denser than cork- 
wood, the pound would still weigh a number of ounces 
more, owing to the magnitude of the body, which gives 
it its increase of weight. 

6. On the sun there would be a much greater differ- 
ence in the weight of objects than on any of the 
planets, as he is thousands of times larger than any of 
them, thousrh he is not denser than water. His matj;ni- 
tude being so great, a body weighing one hundred 
pounds on the earth w^ould weigh twenty-eight hundred 
on the sun, as his attraction is that much greater on 
objects that are on his surface than that of the earth for 
objects on its surface. 

7. It is now evident that the weight of any body, 
whether small or large, depends upon the force that is 
exerted upon it outside of itself; and, in view of these 
results, as manifested by the sun and the planets, how 
])erfect is the adaptation of man to his present abode ! 
The earth attracts him to herself in a degree suitable to 
his physical nature, and the general adaptation of all 
natural objects here below conspires to enhance his 
happiness and enjoyment. 



SECTIO^^ XL. 

€axtlj's gLtmospbcvE. 

1. The globe on which we dwell is in the centre of 
a great ocean of rare, ethereal, fluid matter, the general 
surface of which is supposed to be about fifty miles dis- 
tant from the surface of the earth. This fluid substance 

Questions. — 6. How much will a body tliat weighs a pound on 
the earth weigh on the sun ? Why will it weigh so much more ? 
7. Upon what does the Aveight of a body that weighs a pound on the 
earth depend if transported to any of the planets ? Is our physical 
strength adapted to the amount of attraction which the earth has 
for us ? 

Sec. XL. — 1. What surrounds the earth? "What is its average 
depth ? 



94 earth's atmosphere. 

is called the atmosphere, and is composed principall}' 
of two gases, viz., oxygen and nitrogen, combined in 
different proportions. The former of these gases is 
essential to the health and growth of both animal and 
vegetable life ; and were it not for its presence every- 
where, the earth would be a barren waste. 

2. We inhale it every moment, and by its influences 
animal heat is generated, and the functions of all 
organized bodies stimulated to the accomplishment of 
those ends for which they were designed. In combi- 
nation with other elements, it is the vivifying principle 
in the atmosphere in which we live, and to which we 
are indebted as an agent for the comforts of life.' Though 
it may differ in some of its characteristics from other 
objects in nature, still it has properties and qualities 
that are common to them all. 

3. It is visible if viewed through a body of itself of 
sufficient depth to develop its color : hence the bluish 
tinge that it manifests when observed at a distance. As 
the eye penetrates it, its hues grow deeper, till the whole 
heavens appear as a canopy of azure suspended from 
above. But the attribute of color is not the only one 
that the atmosphere exhibits : it is also possessed of 
weight. This is evident from a number of considera- 
tions. 

4. By taking two vessels of equal weight, and forcing 
more air into one of them with a condenser than it 
ordinarily contains, and then weighing them separately, 
they will be found to differ in weight. This difference 
of weight is the weight of the excess of atmosphere 
that one vessel contains over the other. So, also, if the 
air is exhausted from a vessel (which can be easily done 
with an air-pump), it will diminish its weight. But 
this idea may be more clearly exemplified by the gene- 

QuESTiONS. — Of what is it composed ? Is it essential to the health 
and growth of vegetable and animal life ? 2. What element in it is 
the vivifying principle, oris the generator of animal heat? 3. Is 
the atmosphere visible? What is its color? Has the atmosphere 
weight ? 4. How can this be proved ? 



95 

ral pressure of the atmosphere, which results from its 
actual weio'ht. 

5. It exerts a heavy pressure on all bodies that are 
on the surface of the earth. At the level of the sea it 
is about fourteen pounds to the square inch, and below 
this it is still more. It will balance a column of mer- 
cuiy — as may be seen in the case of the barometer — 
twenty-nine inches high, or a column of water thirty- 
three feet. So great is its pressure even on our bodies 
-that it would instantly crush them, were it not that 
there is an equal corresponding internal pressure out- 
wards, which preserves them from destruction. 

6. Like water, it exerts this force in every direction, 
and always to that degree which corresponds to its 
general weight. If separated, as it is frequently by 
lightning, it dashes together with a report often louder 
than the heaviest explosion. And often, when it is dis- 
turbed by the heat of the sun and other causes, it 
forces the oak of a hundred centuries from its roots, 
and leaves desolation in its path. The particles of 
which it is composed being at liberty to move freely 
among themselves, it enters every habitation, and by its 
gravity it is caused to pervade every crevice and open- 
ing in the earth. 

7. Notwithstanding it may be very much reduced in 
bulk by pressure, still, as a natural body it occupies 
space and retains its original qualities, as before. Not- 
withstanding it may be rarefied by heat so that it 
ascends, yet it satisfies every demand that is made upon 
it by the animal and vegetable kingdom. Though its 



Questions.— 5. Does the atmosphere exert a heavv pressure on 
the surface of the earth ? AYhat is it to the square inch at the level 
of the sea ? How high a column of mercurr will it balance ? How 
high of water ? Why does it not crush us by its pressure ? 6. Does 
it press in every direction, like water ? If separated by lightning, what 
is the result? What effect has it sometimes on the forest? 7. Is it 
capable of being compressed? Is it elastic? Can it be rarefied? 
Does it move from one place to another? By what influence? 



96 REFRACTION. 

currents are numerous and flow often in opposite direc- 
tions, still, as an ocean without a shore, it is indispensable 
in the present economy of nature. 



SECTION XLI. 

Refraction. 

1. When we wish to take observations of celestial 
bodies, we are compelled to look out through the entire 
depth of the atmosphere; and were it not that it is 
transparent and allows the rays of light to pass through 
it, they would be forever invisible. Hence, by their 
own inherent light which they emit, or by the light 
of the sun which some of them reflect, have we arrived 
at our present knowledge of their phenomena. 

2. Their light travels in straight lines, unless diverted 
by some medium through which it passes, or by some 
surface with which it comes in contact. As it cannot 
reach us without penetrating through the whole depth 
of the atmosphere, as a legitimate consequence their 
rays will be bent from their natural course. This is 
called the refraction of light; and its tendency is to 
change the direction of every ray that enters it obliquely, 
downwards, or towards the centre of the earth. 

3. Now, since all heavenly bodies that are visible are 
visible only by the light that emanates from them, and 
since the atmosphere increases in density as the rays of 
light descend into it, they will necessarily deviate farther 
from a straight line as they approach us ; for, according 
to a well-known principle in science, the denser the 
medium through which light will pass, the greater is 



Questions. — 1. Through what transparent substance must we look 
when we wish to see a heavenly object? Through what agency do 
we see celestial objects? 2. Do the rays of light travel in straight 
lines if not disturbed? Do they travel in straight lines through 
a transparent medium? What is this change of direction called? 
Oblique rays are turned in what direction by the atmosphere? 



«f 



REFEACTION. 97 

its refractive power. On account of this deflection of 
the light that reaches us from all of the heavenly- 
bodies that are visible, they are seldom seen in their real 
positions. (See Fig. 23.) 




4. Being always observed in the direction of that sec- 
tion of the ray that is nearest to us, which is always re- 
fracted most, their altitudes are increased, except when 
they are at the zenith,^ where no refraction occurs. 
When in the zenith, their rays fall perpendicularly upon 
the atmosphere, and, consequently, suffer no change in 
direction in coming to the earth. But, as every heavenly 

^ Zenith — is a point in the heavens directly above us. 

Questions. — 3. Is a ray of light refracted more near the earth by 
the atmosphere than in its upper regions ? What results from this 
in viewing heavenly bodies? 4. Are their altitudes generally in- 
creased thereby? When are they not? When a heavenly body 
is in the zenith, is it seen in its real position ? How when seen on 
the horizon ? n 



98 EEFRACTION. 

body that is visible recedes from this point, they are 
more and more displaced by refraction, till when on the 
horizon it amounts to about thirty-three minutes, where 
it is most. This beino; the case, they always appear 
higher in the heavens than they are ; and in relation to 
the sun and moon, they are elevated so much that they 
appear wholly above the horizon when they are actually 
below it. 

5. By this refractive influence of the atmosphere on 
light, the length of the day is increased a few minutes, 
and right ascension^ and declination^ are affected. The 
declination of heavenly bodies is diminished, and their 
right ascension is either increased or diminished, as they 
are either east or west of the point from which it is 
measured. If they are east, it is diminished, and if they 
are west, it is increased, as it is reckoned clear round the 
circuit of the heavens in an easterly direction. 

6. Besides the general efPect of elevating heavenly 
bodies above their true places, refraction appears to 
change them sometimes in their figures. When the 
atmosphere is very dense, both the sun and moon, when 
near the horizon, appear oval in form. They appear to 
be elongated horizontally, or to be shortened in the 
direction of their vertical diameters at least one-sixth 
of their length. This is the result of unequal refrac- 
tion of the atmosphere that intervenes between them 
and us. As it is denser at the horizon than a little 
above it, the rays of light proceeding from the lower 
edges of either of them are refracted more than those 
that emanate from above. This seems to flatten them 
vertically, while their horizontal diameters are not 
sensibly diminished. 

^ Right- Ascension — is the distance east of a given point, and is 
measured on the equinoctial clear round the heavens. 

^ Declination — is distance either north or south of the equinoctial. 



Questions. — 5. "What effect has refraction on the length of the 
day ? What on declination ? What on right ascension ? 6. What 
efiect has refraction sometimes on the figures of the sun and moon? 



TWILIGHT. 99 

SECTION XLII. 

oTfotliqbt. 

1. Twilight is also another phenomenon which de- 
pends npon the agency of the earth's atmosphere. It is 
partly due to refraction, but chiefly to the irregular 
reflection of the sun's rays by the particles of the atmos- 
phere when he is below the horizon. Were it not for 
the power which the atmosphere has of displacing the 
solar lio-ht and scatterins; it in various directions, no 
objects would be visible to us out of direct sunshine. 

2. Differing as it does in its density as we ascend into 
it, and also in its temperature, it is kept in a state of 
constant agitation, whereby the light is constantly 
scattered and diffused. If it was destitute of this power, 
every shadow would be thick darkness, and every place 
out of the sun's immediate presence would be filled 
with the obscurity of night. Through this dispersing 
influence of the atmosphere, the solar light is turned 
from its direct course and diverted to the purpose of 
general illumination. 

3. It is a well-known fact in relation to the laws of 
reflection, that if a ball is dropped in a perpendicular 
line upon a plane surface it will bound up along the 
line that it described in its fall, or if it is thrown against 
a plane surface obliquely it will leave this surface at the 
same angle that it approached it. The same is also true 
in relation to light, unless the body with which it is 
brought in contact is of such a nature as to absorb or 
transmit it. 

QuESTioxs. — 1. What substance produces twilight? lu what 
way? What effect have reflection and refraction on the ravs of 
light? 2. Does the motion of the atmosphere aid in producing 
twilight ? In what way ? Would there be any twilight if there was 
no atmosphere ? What would every shade be ? 3. If a ball falls 
on a level surface, in what direction will it rebound? How if 
thrown obliquely against a plane surface? Does light follow the 
same law if not absorbed or transmitted by the body with which it 
is brought in contact ? 



100 TWILIGHT. 

4. The atmosphere being possessed of the power of 
reflection and refraction, the light of the sun when he is 
below the horizon passes into it obliquely, and is re- 
flected on portions of the earth from which he is con- 
cealed. And owing to the mobility of the particles 
which compose the atmosphere, and the countless direc- 
tions in which they reflect the light, when it reaches us 
it is so intermingled that no specific ray can be observed. 
This commingled, reflected, and less and more refracted 
light is twilight ; and it ceases to be visible when the 
sun is more than eighteen degrees below the horizon, 
in a vertical line. 

5. It differs in different places on the earth in its 
duration, being shortest at the equator and longest at 
the poles. At the equator, where the circles of daily 
motion are perpendicular to the horizon, it continues 
only a little over an hour ; while at the poles it continues 
for nearly four months. Between these points it varies 
less or more in its length as the pathway of the earth 
is inclined more or less to the horizon. 

6. At either pole, where it is daylight for six months 
and night for the same length of time, the twilight is 
very much prolonged. For as the horizon of a person 
at either pole would be only a little above the apparent 
pathway of the sun at any time, and as he never sinks 
more than twenty-three and one-half degrees below this 
horizon, twilight would not be entirely withdrawn for 
more than ten weeks out of the six months of night. 
And, as if Providence designed expressly to relieve the 
darkness that would then occur, the moon is constantly 
above the horizon, and the Northern Lights very fre- 
quently exhibit their fantastic coruscations, turning the 
night almost into day. 

QuESTioxs. — 4. How, then, does this apply to the production of 
twilight? When does it cease? 5. Does it differ in length on 
different portions of the earth? Where is it shortest? Where 
longest? What is its length near the equator? What near the 
poles? 6. For what reason does it continue so long near the poles? 
Is there any natural provision to relieve the darkness at the poles 
when there is no twilight ? What is it ? 



AUEORA BORE A LIS. 101 

SECTION XLIII. 

giurora ^orcalis. 

1. The aurora borealis, or, more properly, the aurora 
polaris,^ is a luminous phenomenon, which frequently 
appears in the heavens to persons that are in high lati- 
tudes, both north and south. It is also called the 
Northern Liii:hts, and sometimes the Northern Mornino;, 
on account of the advantageous position which we have 
in the north for observing it, and from the close re- 
semblance that it frequently has to the dawn which 
ushers in the day. 

2. These lights are more brilliant in the arctic regions 
during the winter season than at any other place or 
time ; and they consist of luminous rays of various 
colors, which converge towards one common point in 
the heavens. At first they commonly appear as a foggy 
cloud near the horizon, and sometimes they continue in 
that state for several hours without any sensible motion. 

3. This mist frequently assumes the form of an arch, 
the convex surface of which soon becomes luminous 
with a pale, uniform light, after which jets and rays of 
light, variously colored, issue forth in wonderful con- 
trast. Increasing in number and brilliancy, they shoot 
up towards the zenith with great velocity, emulating at 
times the vividness of lightning and the colors of the 
rainbow, till at their maximum- splendor they come 
together in a crown of inimitable beauty. 

4. In a moment they begin to fade, and in another 

^ Aurora polaris — polar light. 
2 Maximum — greatest. 



QtJESTioxs. — 1. Are the polar lights ever seen near the south 
pole? By -whom? By what other names are they known ? 2. Where 
are these lights most brilliant? During what season of the year? 
Of what do they consist ? How do they appear at first, and where ? 
3. What shapes do they sometimes assume as they develop them- 
selves? 4. Do thev suddenly disappear and reappear? 

9« 



102 



AURORA BOREALIS. 



they are gone, frequently to reappear in a form appa- 
rently durable and immovable, and yet as evanescent as 
before. Like a ponderous arch, this phenomenon appears 
again. Though composed at first of whitish light, it 
soon begins to manifest its meteoric coruscations.^ (See 
Fig. 24.) Quick almost as thought, these lights break 




Fig. 24, 

out again, without either uniformity of figure or motion. 
With great diversity of color, they move fitfully and 
fantastically over a whole hemisphere in a moment, 
astonishing the spectator witli the rapidity of their 
changes. 

5. Soon they partially subside, and grow dim, as if to 
recuperate, when suddenly they manifest themselves, 
vigorously as before, in places where they had not been 
visible. Tremblino; and vibratino- and sometimes 
uttering hissing and crackling sounds for a few moments, 
they gather themselves into a more definite form, like a 
scroll when it is rolled loosely together. (See Fig. 25.) 
Apparent uneasiness still attending them, they abandon 
both their positions and their forms, and rise up like 
pillars and pyramids, standing on the horizon and sup- 
porting the archway of heaven. 

6. Observed as they have been by persons of scientific 



Coruscations— brilliant flashes of light. 



Questions. — What is said of their motions ? 5. Do they ever 
produce hissing sounds? What special forms do they sometimes 
assume, and wliere do they appear to stand? 6. Are their colors 
ever very brilliant? 



AURORA BOREALIS. 



103 



attainments, who have attempted to describe them, they 
are represented as tinged sometimes with so deep a red 
that the stars appear as if they were dipped in blood, 
and at other times so variegated that they present all 
the colors of the rainbow ; and, as if to add to this 
chromatic^ picture by introducing into it the symmetry 
of architectural beauty, they form their columns, arches, 




Fig. 25. 

cones, pyramids, and spires, whereby they enhance their 
inimitable grandeur, and enchant the eye with their 
splendor. 

7. These lights may be regarded as being within the 
limits of the atmosphere, and in all probability do not 
extend many miles above the surface of the earth. If 
they were outside of the atmosphere and the earth's 
influences, they, like the stars, would appear to have an 
hourly motion westward ; but their altitude and dis- 
tances do not undergo these hourly changes to which 
celestial objects are subject. Neither have they any 
motion in reference to the zenith and horizon, as would 
result from the daily motion of the earth. 



Chromatic — pertaining to color. 



QuESTio>?^s. — Are tliey frequently variegated ? What architecturnJ 
forms do they sometimes assume? 7. Are these lights within the 
limits of the atmospliere? How can this be proved? 



104 AURORA BOREALIS. 

8. From these facts, together with others which may 
be adduced, it is evident that they are associated with 
the atmosphere, or some matter suspended in it, which 
partakers of the diurnal motion of the earth. In further 
confirmation of what has been said, we may also refer to 
the manifest relation which these phenomena sustain to 
the magnetic poles of the earth. They are influenced 
by them ; and it is highly probable that it is terrestrial 
magnetism and atmospheric electricity that actually 
produce them. 

9. When the aurora commences to form, it generally 
rises slowly at first, in the shape of an arch, with its 
middle not in the direction of the true north, but in 
that of the magnetic needle at the place of observation. 
And if its lines of light extend themselves, they cross 
the heavens at right angles to the plane that would be 
formed by a perpendicular line rising from the horizon 
to the vertex of the arch, and the line of the magnetic 
needle as it points towards the magnetic pole from the 
position of the observer. 

10. If they pass the zenith, as they do sometimes, 
they converge to the same plane, and not to the true 
magnetic meridian ; and during very brilliant displays 
of the auroral light the magnetic needle itself mani- 
fests great uneasiness, and is unwilling to settle till they 
in a measure subside. 

11. But the magnetic influence is not alone in pro- 
ducing these wonderful exhibitions of natural grandeur. 
As electricity is always in company with magnetism, 
and as it is known to be a powerful agent in producing 
some very mysterious results, it is highly probable that 



Questions. — 8. Is there any relation between these lights and 
tlie magnetic poles of the earth ? Through what agency is it pro- 
bable that these lights are produced ? 9. Where is the middle point 
of the auroral arch? When the auroral rays of light extend them- 
selves, where do they cross in the heavens ? 10. To what place do 
they converge when they cross ? . What is said about the magnetic 
needle during brilliant displays? 11. Does electricity always 
accompany magnetism, and magnetism electricity? 



SHOOTING STARS. 10*5 

it is the more active and efficient of the two in creating 
these auroral displays. Though naturally invisible, it 
manifests itself under certain conditions by its qualities 
of heat and of light. If it is passed into a partial con- 
ductor, both heat and light are evolved; and if it is 
passed into rarefied air, the latter appears. 

12. Now, as the atmosphere is rarer in proportion to 
its altitude, and as its strata^are in different conditions, 
and its particles always in motion, and the electricity 
unequally distributed through it, the reflection of its 
light might be reasonably expected to be what is wit- 
nessed in the various exhibitions of the Northern Lights. 



SECTION XLIV. 

^Ijooting ^fars. 

1. Shooting stars have been witnessed, probably, in 
all ages of the world, and in every inhabited country. 
They were noticed and considered by astronomers centu- 
ries before the Christian era ; and ever since its beginning 
they have occupied the attention of those who were 
anxious to know more of their character. Though an 
occasional one might be seen some place in the heavens 
almost any clear night, yet their recurrence when they 
appeared in great numbers was generally knowm. 

2. For thousands of years the dates of their more 
brilliant displays have been recorded, and since they 
have been carefully compared, they appear to be con- 
fined to certain months of the year. Since the year 

^ Strata — layers of atmosphere. 



Questions. — Which is supposed to exert the greatest influence in 
producing these lights ? 12. Is the electricity equally distributed in 
the atmosphere ? 

Sec. XLIV. — 1. Are shooting stars of recent origin ? Did they 
occupy the attention of the ancient astronomers ? Did they discover 
their periodic return ? 2. Were their dates recorded ? 



106 SHOOTING STARS. 

seven hundred and sixty-three, anno Domini, and pro- 
bably before that date, more than three-fourths that 
took place occurred in the month of August, whilst 
nearly the whole of the other fourth occurred in the 
month of November. 

3. At intervals of about thirty-three years they gave 
their grandest exhibitions, — which periodical return may 
yet lead to a more definite knowledge than we now possess 
of these somewhat mysterious phenomena. One of the 
most brilliant displays of shooting stars was on the night 
of the thirteenth of November, one thousand eight 
hundred and thirty-three. It commenced about nine 
o'clock in the evening, became strikingly brilliant at 
eleven, increased in splendor till four, and continued, 
with slight variations, till morning. 

4. During this period, nearly the whole of the visible 
heavens was covered with stars, diflPering in size and 
brilliancy, shooting generally in an oblique direction 
towards the earth. Some of them were so small that 
they were scarcely visible, except by the line of light 
that they left behind them as they passed through the 
air. Others were large, luminous bodies, irregular in 
form, and almost stationary till they disappeared; whilst 
a few of them Avere thousands of feet in diameter, 
nearly globular in form, and appeared to light up that 
region of the heavens in which they were, as if com- 
posed of the most brilliant flame. 

5. Nearly all of these large bodies that traversed the 
air would leave a stream of light, more or less bright, 
which in some instances would remain visible for more 
than an hour. These streams of light no doubt ema- 
nated from particles that were detached by the atmos- 
phere as they passed through it ; and they frequently 

Questions, — In what month do most of them occur? 3, What 
is the length of the periods at which they occur? When was a 
great display witnessed ? 4. Did the meteors vary in size? Were 
they all regular in form ? Were some of them very large ? Were 
any of them very brilliant? 5. Did they leave streams of light 
behind them as they passed through the air? 



SHOOTING STARS. 107 

manifested different prismatic^ eolors, the red, yellow, 
blue, and green predominating. Plissing and crackling 
sounds issued from some of them ; and others, of the 
larger class, after traversing the heavens for a consider- 
able distance, would explode with a report loud as that 
of a cannon. 

6. These meteors appeared to emanate from one par- 
ticular point in the heavens, which was located in the 
constellation Leo ; and though many of them shot in an 
easterly and other directions, yet the great majority of 
them described their lines towards the west. This point 
was stationary, and could not have been within the 
limits of the atmosphere, as it w^ould then have had a 
corresponding easterly motion with the earth on its 
axis, but it had an apparent westward motion with the 
constellation in which it appeared to be. 

7. This is universally the case in all those great exhi- 
bitions of meteoric showers that occur: they emanate 
more particularly from some specific point in the heavens 
which is stationary, and are never very far distant from 
where it was on previous occasions. From this fact, 
together with others to which we will hereafter refer, it 
may be inferred that they have their origin outside of 
the atmosphere, and though they may come in contact 
with it, it only ignites them, or adds to their brilliancy, 
as they tend towards the earth. 

8. The height at which many of them manifested 
themselves is not definitely known, neither are the ele- 
ments of which they are composed; but, from their 

^ Prismatic colors — colors that are produced by a prism when it 
decomposes light. 



Questions. — Were these streams sometimes of different colors? 
Did any make any noise? Did any of them explode? 6. From 
what point in the heavens did they appear to issue? In what direc- 
tion did they shoot ? AVas the point from which they issued in the 
atmosphere, or outside of it ? What evidence that it was outside ? 

7. Is there any similarity in all of these periodic displays ? Do the 
meteors enter the atmosphere? What efiect has it on them? 

8. Have we a knowledge of what they are composed of? 



108 METEORITES, AEROLITES, AND FIREBALLS. 

diversity of color, it is probable that they are constituted 
of matter that differs slightly in its nature. As meteors^ 
and meteorites in all probability have one common 
origin, we will refer to them again in the sequel of the 
next subject. 



SECTION XLV. 

Peteorites, gi^rolttes, mxb Jireballs. 

1. Meteorites, aerolites, and fireballs, though differ- 
ing more or less in their manifestations, may be con- 
sidered as parts of the previous subject. Notwithstand- 
ing the fireballs frequently appear here and there over 
the earth, they generally accompany the periodic exhi- 
bitions of shooting stars which we have in a measure 
described. (See Fig. 26.) Some of them are appa- 
rently as large as the moon, and move rapidly in an 
oblique direction towards the earth, generally producing 
some noise in their course, till they finally burst asunder, 
casting their fragments of stony matter often in every 
direction. 

2. These bodies are generally less or more globular 
in form, and vary somewhat in their color and brilliancy 
as they traverse the atmosphere, leaving in their wake 
a stream of luminous matter, which gradually diminishes 
till no trace of it is left. Though these phenomena 
characterize some of them, others totally disappear 
when tliey explode, as if they were composed of gas or 
some rare ethereal substance analogous to it. 

3. Often, when aerolites fall to the ground, there are 
no manifestations of light accompanying them whatever. 

^ Meteor — shooting star. 



Questions. — 1. Do fireballs generally accompany the shooting 
stars when they appear in great numbers ? How large do some of 
them appear to be? Do they generally burst as they approach the 
eirth? 2. When they burst, do they always disappear? 3, Do 
aerolites ever fall to the ground without manifesting any light? 



METEOKITES, AEROLITES, AND FIREBALLS. 



109 




110 METEORITES, AEEOLITESj AND FIREBALLS. 

They visit us, sometimes, unheralded by any celestial 
phenomena, except the hissing noise that they make as 
they fly towards the earth. Again, they are betokened 
by small, dark clouds, in which are a number of suc- 
cessive reports like that of cannon, intermingled with 
that of the drum and the musket. At another time 
the cloud appears like an ashy mist, projected on the 
blue, clear sky, from whence issues a low, murmuring 
noise, accompanied with sounds more distinct, shortly 
after which a shower of meteoric stones descends upon 
the earth. 

4. These fragments are generally very hot when they 
reach it, and are incrusted with a dark substance, as if 
their surfaces were blackened by burning. They are 
of different forms and different weights, varying from 
a few ounces to hundreds of pounds. The whole 
number of simple elements that have been discovered 
in them is eighteen ; but this number is not found in 
any one of them. Most of them are composed princi- 
pally of malleable iron and nickel, having intermingled 
sometimes with them traces of sulphur, copper, and 
carbon. 

5. These elements are not combined in definite pro- 
portions, but partake more of a mechanical mixture, 
formins: masses which are not analoc^ous to anv that 
are known to belong naturally to the earth. The 
character of the iron and nickel that they contain 
clearly manifests that they are not of terrestrial origin, 
as these metals exist in the earth as oxides, and not in 
their metallic form, as they do in the aerolite. 

6. Hence, if they have not their origin in the earth 



Questions. — What kind of noise do they generally make? At 
other times what kind? 4. Are aerolites generally hot when they 
reach the earth ? What is their color ? Do they vary in weight ? 
How many simple elements have been discovered in them ? Which 
of the elements are most abundant in them ? 5, Hoav are these ele- 
ments combined ? What kind of iron do they contain ? What is 
the inference in relation to their origin ? 6, 7. Are they planetary 
in their nature and character'^ 



METEORITES, AEROLITES, AND FIREBALLS. Ill 

or the atmosphere, the inquiry naturally arises. From 
whence do they come? Are they from the moon, or are 
they composed of the tails of comets, some portions of 
which may be detached in their perihelion passage 
around the sun and afterwards condensed? Are they 
constituted of nebulous matter unequally condensed, 
which is supposed by some astronomers to be diffused 
irregularly throughout all space? or are they planetary 
in their nature and general motion ? 

7. These are questions which have presented them- 
selves during the history of these phenomena ; and all 
of them may be regarded as having been satisfactorily 
answered in the negative, except the latter, which 
appears to be, what is claimed for it, an explanation of 
the origin- of all the objects to which we have referred 
in the present and previous section. Though they 
manifest great diversity in their nature and character. 
— some small, others large, some luminous, others non- 
luminous, some apparently gaseous, others solid, some 
noiseless, others producing heavy reports, — still some of 
these manifestations may result from the change of cir- 
cumstances which they experience when they arrive in 
the region of the earth. 

8. Assuming now, what is practically demonstrated 
by the fall of the aerolite, that there are small bodies 
within the limits of the planetary system that are not 
visible to us, may it not be that they revolve around 
the sun, or the moon, or the earth, as these bodies 
revolve around each other? A small body, or even a 
ring or a zone composed of atoms, is as much the sub- 
ject of the centrifugal and centripetal forces as one of 
the planets ; and were their orbits very eccentric, so 

Questions. — May a change in their condition account for some of 
these phenomena ? Is it probable that they revolve around the sun 
or the earth or the moon? 8. Would a ring of atoms be as much 
under the influence of the centripetal and centrifugal forces as a 
planet ? Might some point in the orbit of the earth be near to such 



112 ZODIACAL LIGHT. 

that tliey would cross or come near the pathway of the 
earthj unusual phenomena might readily occur. 

9. If small bodies, or the richer portions of a re- 
volving ring composed of very small bodies unequal in 
size, should cross or come near the ecliptic as the earth 
arrives at that point, its greater attractive force would 
naturally collect some of them to itself Entering on 
new conditions as they leave their orbits, and coming 
in contact with the atmosphere, the compression of 
which has a tendency to expand and ignite them, all 
that we witness in the shooting stars, the meteorites and 
fireballs should not be considered as beyond these causes 
to produce. 

10. Oxygen, which is an element of the atmosphere, 
is the supporter of combustion, and by its action on 
rare and denser bodies which suddenly enter into it 
they are rendered, no doubt, the mysterious and won- 
derful objects that sometimes are seen to fall on and in 
the direction of the earth. 



SECTION XLVL 

^obiHcal WW- 

1. The zodiacal light was discovered between two 
and three centuries ago, and was particularly described 
some time afterwards by Cassini, who gave it the name 
by w^hich it is known, because of its being within the 
limits of the zodiac. When viewed from our latitude, 
it has the appearance of a faint light which is conical 
in form and is inclined a few degrees towards the 
horizon. 



Questions. — 9. If the earth and a zone of small bodies should 
come near together, what would result ? 10. What element in the 
air is the supporter of combustion ? 

Sec. XLVI. — 1. When was the zodiacal light discovered? Who 
gave a particular description of it? What is its appearance as seen 
from our latitude ? 



ZODIACAI. LIGHT. 113 

2. In the torrid zone it appears more brilliant and 
better defined, owing to greater facilities that are afforded 
in point of location and clearness of the atmosphere for 
observino; it. The twilio^ht obscures it when the sun is 
less than eighteen degrees below tlie horizon ; and, as it 
follows the pathway of the earth, it is visible at the 
equator during most of the time, while in either of the 
temperate zones it can be seen only at certain seasons of 
the year. 

3. The most favorable time for observing it outside 
of the torrid zone is in the months of April and May, 
October and November. During the first two months 
it becomes visible in our climate after the twilight, in 
the evening, with it^ base resting on that part of the 
horizon where the sun at this time sets. In the months 
of October and November it may be seen in the eastern 
sky before the dawn, extending upwards in an oblique 
direction, as before ; and throughout the month of 
December it manifests itself before sunrise and after 
sunset. 

4. It appears in form like a pyramid or cone, owing 
to the fact that we see only a portion of its edge, a part 
of it ahvays being concealed. (See Fig. 27.) Its real 
shape is probably that of a double convex lens, the 
circumference of which is supposed to be in the plane 
of the equator of the body around which it revolves. 
It appears to change both east and west in its distance 
from the sun, and also in its length, and in the width 
of its base. 

5. It varies from forty to one hundred degrees in its 
height, and from eight to forty on the plane of the hori- 
zon at its base. But these are not the only changes 



QuESTioxs. — 2. How doe? it appear when seen from the torrid 
zone? For what reason ? How far below the horizon must the sun 
be before it is visible? From what place is it almost constantly 
visible? 3. AVhen is the most favorable time for observing it from 
the temperate zones ? Where may it be seen, and what is its position ? 
4. What is its apparent shape? What is its real shape? 5. Does 
it appear to varv in its height and width ? 

10* 



114 



ZODIACAL LIGHT. 



that this object is known to undergo. It changes 
suddenly in the intensity of its light. These luminous 
variations are very obvious in tropical climates, espe- 
cially on the Andes, in South America. Whether they 




Fig. 27. 

are only pulsations produced by the motion and in- 
equality of density of the atmosphere, like what is wit- 
nessed in the tails of comets, or are coruscations arising 
from physical causes within itself, no one can tell. 

6. Neither is it known whether it is self-luminous or 
shines by reflected light ; but the latter is most probable, 
as it is only under certain conditions and in peculiar 
relations to the sun and the observer that it is visible. 
Uncertainty also exists in relation to the place that it 



Questions. — Does it ever change in the intensity of its light? 
Where are they very obvious ? What produces them ? 6. Is it 
self-himinous ? What is its place in the planetary system ? 



LIGHT. 115 

occupies in the planetary gystem, and whetlier it is in 
motion or at rest. 

7. Some astronomers have supposed it to be nncon- 
densed, nebulous matter in the shape of a ring, which 
revolves in the vicinity of the earth's orbit around the 
sun, in accordance with the laws that control the planets 
in making their annual periods. Others have imagined 
it to be a ring or zone of the same composition, which 
belongs to tlie earth and revolves around it, like the 
rings of Saturn, as she rotates on her axis. 

8. These theories a])pear to harmonize better with 
the character and motions of the various bodies tliat 
are known to belong to the solar system than any other 
that has been presented, as either of them would answ^er 
the conditions and account for the phenomena mani- 
fested by the object no^y under consideration. May it not 
be that there is revolving freely in space, between the 
orbits of Venus and Mars, an annulus of nnsolidified 
matter, wdiich is capable of reflecting dimly the light 
of the sun, and which is the material cause of the 
zodiacal light? 

SECTION XL VII. 

figbt. 

1. The sun is tlie great source of natural light. He 
emits it constantly in every direction, and in such im- 
measurable quantities that the most intense artificial 
light is comparative darkness when contrasted with it. 
It is the universal medium through which we obtain by 
our organs of sight a knowledge not only of the exist- 
ence of objects with which ^ve are not in direct contact, 
but also of some of their properties and qualities. 

Questions. — Does it revolve? 7. What suppositions by astrono- 
mers concerning its real shape and motion, and laws that control 
it? 8. Would either of the:>e theories; account for its phenomena? 

Sec. XLVII. — 1. "What is the great source of natural light? 
Through what medium are we enabled to see objects ? 



116 LIGHT. 

2. They disclose themselves to us, if they are opaque, 
by reflecting the light with which they are surrounded 
in the shape of their own images on the retina of the 
eye, the seat of vision, from which photographic like- 
nesses their forms and character are revealed to the mind. 
In this manner we discern luminous bodies that are at 
a very great distance, as well as non-luminous ones that 
are comparatively near to us ; and though we are familiar 
with the use of this medium through which we see 
them, under various circumstances, yet it is not a mis- 
nomer when we call it a mysterious agent. 

3. Notwithstanding its source is the sun, so meagre is 
the knowledge that we have of its origin and manner 
of its motion that it scarcely transcends the limits of 
theory and conjecture. Some philosophers have ima- 
gined it to consist of exceedingly small particles of 
matter which are constantly being thrown off by the 
sun to the enormous distance at which he is visible. 
Others have supposed it to originate from a vibratory 
motion of the sun's photosphere which produces an 
undulatory motion in an ethereal fluid infinitely rarer 
than air, which is regarded as filling all space, whereby 
we experience the sensation of light. 

4. In support of the former theory, an analogical 
argument is adduced from the method by which we ob- 
tain knowledge of an odor. As it is recognized by the 
material particles which are sent forth by the body that 
produces them to the organs of smelling, in like 
manner visible objects are supposed to send forth 
particles of light which enter the eye, whereby vision 
is produced and perception of things at a distance ob- 
tained. And in confirmation of the latter theory an 
analogy is drawn between the eye and the ear. As 

Questions.— 2. How do opaque bodies reveal themselves to us 
through the medium of light ? Where are their images formed 1 
Is light perfectly understood ? 3, Have we a perfect knowledge of 
its origin and motion ? What are the conjectures of some astrono- 
mers? What of others ? 4. What analogy is adduced in support 
of the former theory? What in support of the latter? 



LIGHT. 117 

the undulations of tlie atmosphere produced by a sound- 
ing body pass through the air to the ear, so are the 
vibrations or undulations of the ethereal medium sup- 
posed to pass through the space intervening between 
the visible objects and the eye that perceives it. 

5. This analogy, though insufficient in itself to satisfy 
the inquiring mind in relation to the truth of the 
principle on which it is founded, still has its weight in 
the scale of moral evidence in favor of the theory in 
question, as no phenomenon has yet been discovered 

lecidedly at variance ^ith what it contains. Xearly 
all of the phenomena of light, instead of being at 
variance with its principles, flow from them with re- 
markable ease, and in many instances consequences 
deduced from this theory have been found to be accu- 
rately true when brought to the test of experiment. 

6. But notwithstanding nearly all of its conditions 
are at least apparently fulfilled, as discovered by analy- 
sis and observation, still it must forever remain what it 
is, a theory, unless nature shall disclose to us facts in 
the case which are now beyond the present acquisitions 
of science. This wave or undulatory theory, as it is 
sometimes called, is the one that is now most generally 
received by scholars of the highest scientific attainments; 
and, that it may be better understood, we shall notice 
more particularly what it assumes, and what is claimed 
as flowing therefrom. 

7. It supposes that there exists throughout all space, 
and in all bodies, an exceedingly attenuated and highly 
elastic fluid, called ether. It also supposes that the 
particles of a luminous body are in a state of rapid 
vibration, and that these vibrations are communicated 
to the ether. And as a consequence of these vibrations, 

Questions. — 5. Which theory should have the precedence ? Do 
the phenomena of light correspond with the latter theory ? Do they 
appear to flow naturally therefrom ? 6. Is the wave theory generally 
received by astronomers as the most scientific? 7. What does it 
assume ? What next ? What as a consequence is supposed to flow 
therefrom ? 



118 LIGHT. 

waves of the ether are propagated from the luminons 
body to the eye, which enter it, and, acting on the nerves 
that constitute the seat of vision, produce the sensation 
of light. 

8. The motion and form of these waves may be 
familiarly illustrated by dropping a pebble or stone into 
a pool of water which is perfectly at rest. Circular 
waves or undulations move off in every direction from 
the point at which the stone entered the water, and 
continue to flow outwards until they are lost on the 
shore, or gradually disappear in the distance by counter- 
acting influences which have a tendency to bring them 
to rest. In these waves the water does not actually 
travel ; it is only the swell that travels : hence the rapidity 
with which they move from one place to another. 

9. This is the method by which we obtain a know- 
ledge of sound. The vibratory motion of the sonorous 
body, acting on the atmosphere, creates undulations in it, 
which go out in every direction, carrying with them 
the notes or sounds that it produces. These waves, 
coming in contact with the organs of hearing, give rise 
to the sensation of sound ; and its tone is attributed to 
the frequency of the aerial vibrations, and its loudness 
to their amplitude. 

10. In like manner is light supposed to be propagated 
from luminous bodies. It comes by a quick succession 
of ethereal waves that intervene between us and the 
body that gives them their motion, and, in consequence 
of the millions of them that impinge upon the eye in a 
moment of time, its regularity and uniformity are con- 
stantly maintained. So rapidly do they follow each other 
that the eye is not capable of observing the time that 



Questions. — 8. How are the motion and form of the ethereal 
waves or luidulations illustrated ? What travels in these waves, — the 
water, or the swell ? 9. How is sound communicated to the ear ? 
What is the tone attributed to ? What the loudness? 10. Is light 
supposed to travel in a similar manner? What is supposed to give 
it regularity and uniformity ? Are the ethereal waves very rapid in 
their movements ? 



COLORS OF LIGHT. 119 

elapses between tliem, neitlier of discerning any periodi- 
cal dimness of impression in the object that it has 
attempted to beliold. 

11. The impression of an object on the eye, after it 
is removed, lasts abont the one-seventh part of a second : 
consequently, if an object was turned out of and into our 
presence say every tenth of a second, it would be con- 
stantly visible, as if no motion had been given it. 
Hence, if a few ethereal undulations, comparatively, 
should occur every second of time, the light of bodies 
from which it emanates would not be periodical ; but, as 
millions of them reach us in that length of time, the 
light that we daily enjoy can never fail to be steady, 
and will always appear as we see it. 

12. Though the sunlight does not appear at all times 
uniformly brilliant, yet its changes cannot be attributed 
to any variation in the mode by which it is propagated. 
The atmosphere through which it must pass before it 
reaches the earth often turns it out of its course, and 
the clouds and mists frequently obscure it by intercept- 
ing its rays. 



SECTION XLVIII. 

Colors of ITig^t. 

1. Light, according to the best authorities, is com- 
posed of three different colors, — red, yellow, and blue ; 
and all other colors are considered as being formed by a 
union of two or all of them in difi'erent proportions. 
The natural appearance of light is white ; but if a ray 

Questions. — IL What length of time does the impression of an 
object last on the seat of vision after it is removed? If the ethereal 
waves were more than seven per second, would the light be steady 
and uniform? Are there millions of them per second impinging 
upon the eye? 12. Does the sunlight appear at all times uniformly 
brilliant? Is it dimmed and brightened by the mode in which it is 
propagated ? What changes it ? 

Sec. XLVIII.— 1. How many primary colors are in light? What 
is the natural color of liii'ht ? 



120 



COLOKS OF LIGHT. 



of it is permitted to pass through a trianguhir piece of 
glass, called a prism, seven different colors may be seen, 
which correspond to the colors of the rainbow. (See 
Fig. 28.) 

2. This white ray is both refracted and decomposed, 




Fig. 28. 

and each color leaves the prism, diverging not only from 
the original ray of which they are elements, but also 
from each other, as may be seen by observing the spec- 
trum^ which they form. This spectrum exhibits these 
colors in the order of their susceptibility of refraction, 
the red being refracted least, and the violet most. 
Judging from its appearance, it might be inferred that 
there were actually seven primary or distinct colors in 
the composition of light ; but, by a most critical investi- 
gation and analysis, it has been determined otherwise. 
The orange, green, indigo, and violet are discovered to 
result from a blending of the primary colors in different 
degrees of intensity as they form the spectrum. 

3. If a ray of red liglit is peruiitted to fall obliquely 

^ Spectrum — different colors that proceed from a prism, a? seen on 
a plane surface. 

QuE&TiONS. — How is lig-ht decomposed? 2. What two effects has 
the prism on light? How does the spectrum exhibit these colors? 
"Which color is refracted most? Which least? How many colors 
does the spectrum exhibit? How many are simple colors? How 
mnny are compounds? 3. What effect has the prism on a red ray 
of light that eiUers it obliqnely? 



COLORS OF LIGHT. 121 

on a prism placed in a small, circular hole in a wall, it 
will be refracted by it, and pass out of it not as it entered 
it, in a round form, but it will extend the whole length 
of the seven-colored spectrum. This ray will have a 
greater intensity at some certain point on the spectrum, 
and will grow gradually dimmer in either direction, till 
it fails to be perceptible. The results are the same in 
relation to the yellow and blue rays, if they are dealt 
with in the same way : each one covers the whole spec- 
trum which would be formed by the seven colors when 
they reach it, though differing, like the red color, at 
every point in its intensity. 

4. From these and similar experiments it has been 
ascertained that light is a compound, the elements of 
which alone give beauty of color to tha various objects 
with which we are surrounded. The natural color of 
an object when exposed to sunlight depends upon the 
peculiar nature and arrangement of the particles of 
which it is composed, and not upon any inherent quality 
in itself. 

5. If a body appears red, or blue, or yellow, it is not 
because it is naturally so, but because it is so constituted 
as to reflect the red, or blue, or yellow elements of the 
light, and dispose of the remaining ones either by 
absorption or transmission. When bodies appear white, 
they reflect the light just as they receive it, because 
they have no preference for reflecting one of its elements 
more than another. But if they appear black, their 
molecular arrangement is such as to absorb all of the 
light without reflecting any of its colors. 

6. In relation to bodies that are neither white, nor 
black, nor red, nor yellow, nor blue, but exhibit some 

Questions. — Has tliis ray a greater intensity on the spectrum at 
one point than another ? Is it so with the yellow and blue rays ? 
Does each ray cover the whole spectrum? 4. From the above facts, 
would we infer that light is a compound? On what does the color 
of an object depend when exposed to sunlight ? 5. Why does a body 
appear of any particular color ? When a body appears white, what 
eifect has it upon light ? When black, what effect ? 6. Does it differ 
in its molecular arrangement when it reflects a varietv of colors ? 

11 



122 COLOES OF LIGHT. 

other color, or are variegated, their nature is such as to 
absorb or transmit some of the colors, or a less or greater 
portion of them, while other colors, or portions of them, 
are reflected to the eye of the observer. 

7. Besides these various conditions in nature, whereby 
a great variety of colors are constantly manifested by 
natural objects and objects treated artificially, there are 
other colors, also, that are produced by contrast or by 
bringing one in juxtaposition to the other. Every 
color placed beside another color is changed, and appears 
differently from what it does when seen alone. They 
are either rendered dimmer or brighter, or some of 
them are changed apparently from one color to another. 

8. If a white color is placed close to a red one, and 
observed at a (Jistance, the white will appear bluish 
green ; or if a white color is placed near a blue, the 
white will apparently change to orange. White con- 
trasted in the same way with yellow changes to indigo ; 
with green, to reddish violet ; with blue, to orange red ; 
with indigo, to orange yellow ; and with violet, to yel- 
lowish green. 

9. Colors that are produced in this way — that is, by 
bringing one near to another — are called complementary ; 
and we are less or more influenced by them every day, 
in the color of the clothing that we wear, the enchant- 
ing scenery of the landscape, and the variegated hue? 
that everywhere adorn the world with beauty. 



Questions. — 7. Are colors produced in any other way than by 
the nature of the objects that reflect them? In what other way? 
Does one color appear to change another by bringing them near 
together ? 8. When white is brought near to red and viewed at a 
distance, what is the effect ? When white is placed near blue, what 
then? White near yellow ? White near green ? White with blue ? 
White near indigo ? White near violet ? 9. What are these new 
colors called ? Are we influenced by them every day ? 



REFKACTING TELESCOPE. 123 



SECTION XLIX. 

^cfriuting telescope. 

1. It is a well-known fact that some substances will 
transmit light, or allow it to pass through them. The 
rays of the sun penetrate through the whole depth of 
the atmosphere in coming to the earth ; and if they 
enter it obliquely they will constantly change their 
direction a little as they approach us. So also is it 
when they come in contact with glass ; they are changed 
in direction as they travel through it, by the refractive 
influence which it has upon them. 

2. When parallel rays of light enter a piece of glass 
that is concave on both sides, they are dispersed, or 
rendered divergent ; but if they enter a piece of glass 
that is convex on both sides, they are collected or caused 
to converge to a point, called a focus. The latter kind 
of glass in form is generally used in the construction of 
the refracting telescope, and is called a double convex 
lens. 

3. For telescopes which are employed in viewing 
heavenly bodies, only two lenses are used ; but those 
used for observing terrestrial objects generally have four. 
This latter number is necessary only when we wish to 
see the objects not inverted ; but in the case of heavenly 
objects the observer is indifferent as to the relative place 
of any of their parts, if their general aj)pearance re- 
mains unchanged : hence the use of only two. 

4. The telescope with only two lenses, one eye-glass 
and one object-glass, is the most simple in form, and is 
generally used for astronomical purposes. It is made 

QrESTioxs. — 1. What substances are transparent? Do trans- 
parent substances change the direction of the rays of light, if they 
meet them obliquely ? "What kind of substances change the direction 
of the rays of light most? 2. "What efiect has a double concave 
lens on parallel rays ? What effect has a double convex ? Which 
kind is used in the refracting telescope ? 3. How many are used in 
some? How many in others? 4. Which is the most simple? 
Which is used for astronomical purposes ? 



124 



EEFEACTING TELESCOPE. 



by placing two double convex lenses in a tube^ at a dis- 
tance from each other equal to their two focal distances, 
so that if an observation is taken of a star, or planet, or 
any other heavenly body, its image is formed at the 
focal point of either glass. This being the case, the 
image is then magnified by the eye-glass, and this in 
eifect is brought near the eye. 




Fig. 29. 

5. The telescope with four glasses is called some- 
times the double telescope ; and the three lenses nearest 
the eye are called eye-glasses, and the one furthest from 
the eye is called the object-glass. The eye-glasses are 
placed at equal distances from each other, and these 
distances, respectively, are equal to double the focal 
length of each glass : so that when the image of an 
object is formed in the telescope by the object-lens, it is 
transmitted through the other three lenses to the eye. 

6. The two middle lenses placed between the object- 
and eye-glasses in the refracting telescope are employed 
to change the direction of the rays of light as they 
travel to the eye, so that the observer may see the 
images of objects erect and not inverted. This kind of 
optical instrument may be properly named a terrestrial 
telescope, as it is generally used for viewing objects that 
are on the earth. (See Fig. 29.) 



Questions. — Where do we place the glasses ? At what distance 
apart ? Where is the image formed ? What magnifies it ? 5, What 
are four-glassed telescopes called ? How are the glasses named ? 
6. What is the use of the two middle glasses ? What might this 
kind of instrument be properly called ? 



REFLECTING TELESCOPE. 125 

SECTION L. 

Reflecting S^elcstope. 

1. The plane mirror which is in general use is 
employed for the purpose of reflecting the image or 
likeness of the object that is placed before it. If 
parallel rays of light fall perpendicularly upon it, they 
are reflected back in lines which are perpendicular also 
to its surface. 

2. Mirrors with convex surfaces disperse or cause 
parallel rays of light that fall upon them to diverge as 
they are reflected ; while concave mirrors cause parallel 
rays of light that fall upon them to collect or converge 
to a point as they are reflected from their surfaces. The 
action of each kind of mirror is uniform in its effects 
on light, it matters not of what it may be composed, 
providing it is well adapted to the purpose for which it 
is designed, and properly constructed. 

3. For the reflecting telescope two concave mirrors 
are frequently employed ; and they are generally com- 
posed of highly-polished metal. These reflectors, a 
small and large one, are placed in a tube some distance 
apart, which is open at the end most distant from the 
eye. The large reflector, or speculum, has a hole in its 
centre, and is placed nearer the observer than the small 
one, with its convex surface in the direction of the large 
end of the tube. 

4. The small reflector is placed near the open end of 
the tube, with its concave surface facing the concave 
surface of the large one. In the small end of the tube 
a plano-convex lens is placed, which magnifies the image 

Questions. — 1. What is a plane mirror used for? How are 
parallel rays of light reflected back ? 2. What effect have convex 
mirrors on parallel rays of light that fall upon them ? What have 
concave mirrors? 3. Hoav many mirrors are used for reflecting 
telescopes? In what are they placed? What is the- hole in the 
large mirror for? Of what are the mirrors generally composed? 
4. How are these two placed in the tube? Where is the lens 
placed? 



126 REFLECTING TELESCOPE. 

which is formed in the telescope by the small reflector, 
by increasing the visual angle. (See Fig. 30.) 

5. When the observer wishes to use this instrument, 
the larger end is directed towards the object to be 
examined, and the rays of light coming from it pass 

I 



»jr 



^ 



i 



4 



Fig. 30. 



down the tube and fall first on the large reflector. From 
thence they are reflected upon the small mirror, which 
reflects them back again, and forms an image of the 
object between itself and the lens that is in the smaller 
end. As the rays of light that bear the image pass 
through the lens to the eye, it is magnified by the lens, 
and makes the vision also more distinct, as the distance 
in effect to the object is shortened. 

6. Telescopes in general have many different forms, 
notwithstanding they are all constructed on one princi- 
ple, which is that of the human eye ; and through their 
discovery new themes and subjects for the contemplation 
of the astronomer have been opened up, which beckon 
him on to the investigation of worlds and schemes and 
systems of heavenly bodies which were hitherto un- 
known. 

7. By their use the moon has revealed its true nature 
and character. Jupiter has been found to be a con- 



QuESTiONS. — For what purpose is it used ? 5. How is this instru- 
ment used? What is the use of the hirge reflector? What the 
small ? What the lens ? 6. Are telescopes of different forms besides 
tlie two described ? On what principle are they constructed ? Have 
tliey been of great use in astronomical discovery ? 7. Name some 
of the objects that were discovered by the telescope. 



CO.MET.S. 127 

trolling centre around which circulate a number of 
satellites. Saturn has portrayed to us a series of rings 
and moons. Uranus and Neptune, with their subordi- 
nate worlds, have come into view ; and more than eighty 
asteroids, which belong to the solar system, have asserted 
its descrying power. So also have the starry heavens, 
and the nebulse, and the Milky AYay, Avhose enchanting 
grandeur, when viewed with aided vision, baffles all de- 
scription, borne testimony to its wonderful revelations. 



SECTION LI. 

Cowtts— Sbcir (Karig Jistorg. 

1. The early history of comets, like that of the 
grouping of the stars and discovery of some of the 
planets, is involved in much obscurity ; but that they 
were known to exist hundreds of years before the 
Christian era, is fully attested. In the literary records 
of the Greeks, Romans, Chinese, and other ancient 
nations, they are mentioned with other heavenly bodies ; 
and the constellations through which some of them passed 
were at least circumstantially noticed. 

2. xlt the extreme end of the thread of their history, 
and for thousands of years afterwards, whatever inform- 
ation was obtained concerning them appears to have 
been that which could be known only by the naked 
eye, and not that which would result from a studied 
analysis or mathematical calculation. 

3. Their relation to the solar system was unknown. 
None of the elements of their orbits were determined. 
The causes of their near approach to us were un- 

QuESTiONS. — May others still be discovered by their use ? 

Sec. LI. — 1 . Is it known when comets were first observed ? Were 
they known to exist centuries before tlie Chi'istian era ? By what 
nations? 2. Early in their history, what was known concerning 
them ? 3. Was it known that they belonged to the solar system ? 



128 COMETS. 

accounted for; and their mysterious forms sometimes 
filled the world with dread, and sometimes with reve- 
rence. Whence they came, or whither they went, were 
problems unsolved; and whether they belonged to the 
material or the spiritual world was often the subject of 
serious conjecture. 

4. But this condition of things was not always to 
exist. Intellect has been gradually unveiled, and the 
lawless wanderer has been discovered to be a loyal sub- 
ject. The apparent fugitive has been found to travel 
in the pathway assigned it, thereby fulfilling the end 
of its existence in the economy of nature. 

5. The erratic comet, so long appalling in its advent, 
and nearly as much so in its departure, is no longer the 
disloyal subject, abandoned to some unknown and 
mysterious fatality, but, like the kind visit of a friend, 
it is known to return from the invisible depths of space, 
to greet us with its presence. (See Fig. 31.) 

6. Though it is not positively known that they all 
return through the attractive influence of the sun, yet, 
from what we know of many of them, we may infer that 
nearly all are members of the solar system. If it could 
be accurately determined to which of the four great 
conic curves, sections of their orbits nearest to the sun 
belong, at once the problem would be solved. 

7. If they travel in circles or elliptical curves, their 
return is certain, no matter how long they may be 
absent or to what distance they may go. But if they 
move in curves possessing other properties, they will 
depart from our system forever, unless arrested by some 

Questions. — Were any of the elements of their orbits determined ? 
Was it known from whence they came, or whither they went? Had 
the ancients a knowledge of what they were? 4. Have astronomers 
obtained a more definite knowledge of them ? 5. Have tliey been 
found to be subjects of natural laws ? What two forces control them ? 
6. Do they all revolve around the sun in certain periods ? If it was 
known in what kind of curves all of them travelled, what could be 
determined ? 7. If (hey travel in circles or elliptical curves, is their 
return certain ? If they travel in curves with other properties, what 
*hen ? 



encke's comet. 129 

other cosmical body within whose sphere of attraction 
they may happen to enter. 

8. Though these curves necessarily deviate from a 
straight line, yet they are as immeasurable as space 
itself; and, were a heavenly body to enter on one of 
them, eternity alone could tell the history of its journey. 




Fig. 31. 

This may be the case with the orbits of a few of the 
comets : still, the orbits of the great majority of them 
resemble a hoop that is pressed in on the opposite sides, 
and, consequently, lengthened. 



SECTION LII. 

^ncke's Comet. 



1. The orbit of the body which is known as Encke's 
comet lies inside of the orbit of Jupiter, and its length 

Questions. — 8. Are the latter kind of curves as immeasurable as 
space? Wliat is the form of the orbits of the great majority of 
them? 

Sec. LII. — 1. Where is the orbit of Encke's comet situated? 



130 

is about double its breadth. In like manner is it with 
the orbits of five other comets, which do not extend 
beyond the outermost planet ; and it may be the case 
with the orbits of others, which have not yet been 
discovered. 

. 2. Encke's comet, to which we have just referred, 
was discovered by Professor Pons, in 1818 ; and subse- 
quently the elements of its orbit were calculated by 
him whose name it bears. It revolves around the sun 
in about twelve hundred days, nearly in the plane of 
the solar system. Its general appearance is that of a 
mass of nebulous vapor, so transparent, even at its 
densest part, that small stars may be seen distinctly 
through it. (See Fig. 32.) 

3. Nothing peculiar has been discovered to belong to 
this body, except that its orbit is constantly growing 
less, and, consequently, its yearly revolutions becoming 
shorter. To account for these phenomena. Professor 
Encke, after having exhausted all causes known to 
exist in the solar system, announced the theory of a re- 
sisting medium. He took for granted that there is an 
extremely subtle, ethereal fluid, which pervades the infi- 
nitude of space, and notwithstanding it is so refined as 
to be almost spiritual in its nature, yet to bodies of such 
extreme tenuity as comets it offers a resisting influence 
which may be perceptible. 

4. This theory was no doubt suggested by the undu- 
latory theory, employed to account for the phenomena of 
optics ; and whether it is the true cause of shortening 
the annual periods of this comet, no one can tell ; but 
I think it should be received with great distrust. 

5. If we admit the existence of such a repelling 



Questions. — What is its length to its breadth ? Are the orbits of 
any other comets entirely within the orbit of the outermost planet? 
2. When was Encke's comet discovered? What is the length of 
its period ? What is its general appearance ? 3. What is said of its 
orbit? How did Encke account for this? What did he take for 
granted ? 4. What suggested this theory ? Should this theory be 
received with distrust ? 5. Why ? 



131 

force, what would be the result ? The centrifugal force 
of every body in the solar system would be weakened, 
and the centripetal force increased, till in the course of 
ages the satellites would fall upon their primaries, and 




they, in turn, upon the sun, destroying forever the 
harmony and order of this great brotherhood of worlds 
with which we are surrounded. 

6. Though this catastrophe might be long delayed, 



Questions. — What effect would tMs medium liave on the centri- 
fugal force? What would, through time, result from its gro\nng 
weaker ? 6. Would such a result betray the wisdom of the Creator 
i*i the construction of the solar system or universe ? 



132. biela's comet. 

yet it would be inevitable ; and the instability of the 
works of a Divine hand would betray the wisdom of 
the Great Author of the universe. But this is not 
to be anticipated; for the history of the universe 
shows that w^hatever apparent irregularities have oc- 
curred in nature, it has always maintained its har- 
mony, which evinces that it possesses the elements 
of its own stability. 



SECTION LIII. 

§i£la's Comet. 



1. BiELA^s comet, like Encke's, is a telescopic object, 
and in some respects is similar, and in others very dis- 
similar, to it. It does not appear to have any nucleus,^ 
or to contain any solid substance; neither to indicate 
that it is composed of any thing except mere vaporous 
matter of exceeding tenuity. 

2. This was evident from its passage over groups of 
fixed stars without concealing them from view. Though 
it was fifty thousand miles in diameter, yet its trans- 
parency was such, even at its centre, that they were only 
slightly dimmed, and then not so much by the inter- 
vention of its matter as by the contrast of its light with 
theirs. 

3. It is one of the six interior comets which are 
known not to go beyond the limit of ISTeptune's orbit, 
neither to deviate far from the general plane of the 
solar system. In consequence of it being so near the 

1 Nucleus — the densest part, or head. 

Questions. — 1. Is Biela's comet a telescopic object? Has it a nu- 
cleus ? Of what does it appear to be composed ? 2. From what is 
this inferred ? What was its diameter ? What caused the stars that 
it passed over to appear slightly dimmed ? 3. In what direction 
does the plane of its orbit lie ? Were any fears entertained that i| 
would come in contact with the earth ? 



1 



133 

planets, fears have been entertained, at times, that it 
would come in collision with the earth and shatter it to 
pieces, or arrest it in its course. But science has dissi- 
pated these gloomy apprehensions, by informing us that 
it has never been nearer to the earth than fifty millions 
of miles, and probably never will be nearer, unless 
when summoned up at the end of time to attend the 
final conflagration. 

4. This comet manifested no peculiar changes till 
recently, when it was observed to separate into two 
parts, unequal in their size and also in their brilliancy. 
(See Fig. 33.) This discovery was first made by Pro- 
fessor Herrick, in 1845 ; and in 1848 it was fully con- 
firmed by other astronomers, in Euroj^e and America. 

5. It was observed to increase in width, and present 
points near either side more luminous than that part 
which lay between. In time, these luminous parts 
diverged, and still continued to diverge, till, when seen 
in 1848, they were one million of miles apart, and when 
last observed they were more than one million and a 
half miles apart. 

6. Though this body had separated, even the parts 
did not cease to change. They alternate in size and 
brilliancy, at one time one preponderating, and at 
another time the other, thereby strengthening the 
opinion of some astronomers, that planets, and other 
bodies considered opaque, do not shine alone by bor- 
rowed light. If they do, how shall we account for the 
bright and dimmer shades of light which they some- 
times exhibit, when change of distance from the sun 
cannot be the cause ! And how also shall we account for 
the dividing asunder of the original body ? 



Questions. — Has science dissipated these fears ? How near has 
it been to the earth ? Is it probable that it ever will be nearer ? 
4. What is peculiar in this comet ? Who made this discovery ? By 
whom was it confirmed ? 5. What took place at first ? Did il divide 
into two parts ? How far apart were they in 1848 ? 6. Did the parts 
themselves undergo any changes ? Have bodies that are considered 
opaque anv inherent light ? 

]2 



134 



BIELA'S COMET. 




Fig. 33. 



135 



7. Many theories have been suggested by which an 
attempt has been made to explain this wonderful phe- 
nomenon ; but we shall have occasion to refer to them, 
in connection probably with the true cause, in the 
sequel of this subject. 



SECTION LIV. 

Jalkg's Comet. 

1. The next extraordinary comet which claims our 
attention is Halley's, a faithful representation of which 
you may see by observing Fig. 34, as drawn by Pro- 
fessor Struve in 1835, when it was last in view. It 
derives its name from the name of the person who 
calculated the elements of its orbit and determined 
the periods of its revolution. They are about seventy- 
five years in length ; and it can be traced back with 
certainty through nine of them, and with much pro- 
bability through a period of more than two thousand 
years. 

2. Before the Christian era it was observed, and was 
equal to a star of the first magnitude, accompanied 
with a tail of wonderful dimensions ; but in later times 
it appears to have decreased both in size and brilliancy. 
At its last return it appeared to be constantly under- 
going changes, and also to oscillate in its orbit, as the 
feathered end of an arrow is sometimes observed to do 
in its hasty flight. 

3. At one time rays of light would dart out in front 
of its nucleus, and turn back on either side, like hair 
which is supported by the wind. At other times the 
nucleus would grow dim, and the tail, at the most dis- 
tant end from the body, would divide through one-third 

Sec. LIV. — 1. From whom did this comet take its name? What 
is the length of its periods ? 2. When was this comet first observed ? 
What then was its size ? Had it a tail ? Has it undergone any 
change ? 3. What manifestations were made by its nucleus ? 



136 



HALLEY'S COMET. 




Fig, 34, 



THE COMET OF 1744. 137 

ol its length into three unequal parts, each terminating 
in a point. 

4. Again, it might be seen with a double emanation, 
one in the direction of the sun, and the other opposite, 
with its outer end turned to one side as if blown by the 
wind. But, wonderful as these transformations were, 
they were no more so than other peculiarities with which 
it was attended. Contrary to the general motion of the 
planets, it revolves from east to Avest, sweeping beyond 
the known limits of Neptune's orbit five hundred and 
fifty millions of miles, into the monotonous solitudes 
of space. 

5. Such is the journey and such is the journeying of 
this wanderer. Solitary and alone, it speeds its endless 
way, satisfying the demands and obeying the will of 
Him Avho knows its end from its beginning. 



SECTION LV. 

'he Comet of 1744. 



1. There are two other comets which demand our 
attention before we proceed to discuss the general cha- 
racteristics which are common to this class of heavenly 
bodies. The comet of 1744 is one of them. When 
first observed, in 1743, it was globular in form, and 
was entirely destitute of a luminous tail. 

2. In 1744 it seemed to change in form and increase 
in splendor, till it rivalled the brightest star and also 
the brightest planet, — when it became visible in the pre- 
sence of the sun, even at mid-day. As it approached 
the centre of our system, it lengthened nearly in the 



Questions. — What by its tail? 4. Had it ever two tails? In 
what direction does it revolve? How far beyond Neptune's orbit 
does its orbit extend ? 5. Is it accompanied by any other body ? 

Sec. LV. — 1. What was the appearance of the comet of 1744 when 
first observed ? 2. What changes did it undergo in 1744? Was it 
visible at noondav ? 

12* 



138 THE COMET OF 1744. 

direction of the line of its orbit, and a dark streak ap- 
peared to divide it longitudinally. From one of these 
parts issued brilliant spires of light, and outside of 
them were semicircular lines of light, alternating with 
dark spaces between. 

3. Soon these odd phenomena disappeared, and the 
comet returned to its original form, but only to undergo 
a far more mysterious and wonderful transformation. 
Apparent uneasiness attended it ; and shortly afterwards 
it exhibited in miniature the figure which it in a few 
days assumed. 

4. Dilating in its hinder part, or the one opposite to 
the sun, and becoming more elongated, it separated into 
six distinct branches behind its head or nucleus. (See 
Fig. 35.) These branches were short at first, but con- 
tinued to increase in length as the comet came nearer 
to the sun. When at the nearest point to him, they 
were at their greatest length, and they manifested phe*- 
nomena never witnessed before or since. 

5. Dark spaces intervened between these streams of 
light, and sparks resembling fire -travelled back and 
forth in their most luminous parts. These sparks grew 
dim and bright as these branches changed in color, till 
after the comet began to urge its flight away from the 
source of heat and light. 

6. Such were some of the manifestations of this 
anomalous body ; but, as they shall recur again, we will 
now proceed to examine the peculiarities of another 
comet, which appeared in 1843. 



Questions. — As it approached its perihelion, what were its 
changes ? 3. Did the comet return to its original form ? Did it re- 
main so ? 4. What form did it shortly assume ? Did the tranches 
increase in length ? When were they at their greatest length ? 5. 
What peculiar manifestations did they exhibit ? Did the sparks grow 
dim and bright? Did the branches change in color? 6. Were 
these manifestations usual for comets ? 



THE COMET OF 1744. 



139 




140 THE COMET OF 1843. 

SECTION LVI. 

^t €omtt of 18-13. 

1. It was visible for forty days, during which time it 
underwent many changes in magnitude, form, and color. 
When first observed, its appearance to the naked eye 
was that of a luminous, globular body, shining with a 
mild but brilliant light even at noonday. Shortly 
afterwards a train commenced to go out from the head, 
possessing a paler hue; and the nucleus itself, at times, 
became partially obscured by a misty envelope, which 
appeared occasionally to surround it. 

2. It also assumed another color, changing from a 
grayish white to a fiery red, and afterwards changing 
back again to the color it was before. At its last return 
its atmosphere nearly grazed the sun, and its velocity 
was at least five hundred thousand miles per hour; 
while that of the outer extremity of its tail was four 
millions of miles per minute. During this rapid flight 
its whole body was manufactured, by the heat and 
power of the sun, into tail, till it was over one hundred 
millions of miles in length. (See Fig. 36.) 

3. Probably no body that has visited our system has 
ever experienced the same degree of solar heat, or ever 
will again, — as it has bid adieu to us forever, to speed 
its way to new immensities ; for as it receded from the 
sun it entered upon the arm of a curve from whence 
there is no return. 

4. Is not this an astounding thought, — never to re- 
turn ? Yet can we doubt the laws of force and motion ? 



Questions. — 1. How long was the comet of 1843 visible? What 
was its form when first observed ? Was it visible in the daytime ? 
Did it soon acquire a tail ? What is said of its nucleus ? 2. Did it 
change its color ? Did it come near to the sun ? What was the 
velocity of its nucleus? What of the extremity of its tail? Did 
its body change into tail ? What was its length ? 3. Has it left the 
solar system, never to return ? 4. Are the natural laws of force and 
motion uniform in relation to heaven] v bodies? 



THE COMET OF 1843. 141 

Can we doubt the harmony of the infinite wisdom that 
directs and omnipotent power that urges it on in its 




Fig. 36. 

flight ? Our reason intuitively decides in the negative, 
and our moral natures answer, No. 



142 FORM AND PROPERTIES OF COMETS. 

5. But are we done with mysteries profound ? Others 
are still suggested by it to our view. On what message 
was this flaming wanderer sent? or to what distant 
world does it now wing its way ? What is its office in 
the economy of nature ? or what page in the book of 
the universe does it disclose ? 

6. These are questions at which reason falters and 
the imagination itself is confounded. Science staggers 
in their contemplation, and points the bewildered mind 
onward to the light of another world, for their solution. 



SECTION LVII. 

§zntxnl Jorm anb ^xoi^txtm of Cometh. 

1. Having noticed some of the peculiarities of a 
few of the most extraordinary comets that have made 
their appearance, we will now consider those that are 
most common, and speak of them in general. (See 
Fig. 37.) They are small, comparatively, and nearly 
globular in form. Their light is of a grayish white, 
gradually diminishing from the centre in some, and in 
others fading away in a similar manner, but in con- 
centric^ waves or rings. 

2. They often change in the intensity of their light: 
yet their variations are never so great as to cut them 
off from the most simple type to which they all belong. 
In ages past they may have been accompanied by lumi- 
nous tails ; but by the waste of time, and contending 

^ Concentric ring — is one that is equally distant from a centre in 
all its parts. 

Questions. — 5. Can we imagine the design of this body ? What 
can be its office in the economy of nature? 6. Can we answer these 
questions with any degree of certainty ? 

Sec. LVII. — 1. Are comets in general comparatively small? 
What is the color of their light ? How does their light appear in 
form? 2. Do they change in the intensity of their light? Are 
their changes so great as to cut them off from the most simple type? 



FORM AND PROPERTIES OF COMETS. 143 

of nature's forces, they may have been driven off, 
leaving the more solid parts in globular form. 

3. About seven hundred comets have been discovered, 
and the elements of the orbits of nearly one hundred 




Fig. 37. 

of them determined. They naturally divide themselves 
into three classes, by a difference in their constitutions, 
and their effects on light. The first have no nuclei,^ and 
are uniformly transparent, permitting luminous objects 
to be seen through them without the slightest dimi- 

1 Nuclei — th.e plural of nucleus, whicli has been defined. 

Questions. — Had they ever tails? 3. How many comets have 
been discovered ? The elements of how many of their orbits have 
been determined? Into how many classes do they divide them- 
selves ? On what is this division founded ? What is said of the 
first class ? 



144 GEAVITY OF COMETS. 

nution of their light. The second class has nuclei ; and 
they permit light to pass through them with equal 
facility. 

4. The third class have star-like centres, differing in 
magnitude and density. The nuclei of only two of 
this class are well defined and truly planetary in form, 
while those of all the rest resemble brilliant points of 
light. Though these classes differ in some respects, still 
they manifest various characteristics which are common 
to them all. 



SECTION LVIII. 

(irabitg of Comets. 



1. Comets are not, as some suppose, the result of 
reflection ; neither are they of an auroral nature ; but 
they have a real, material existence. This fact has been 
established by observing their strict obedience to the 
laws which regulate the planets and other heavenly 
bodies in their annual motions. They obey these 
common laws, which prevail throughout the boundless 
empire of the material universe, and return according 
to prediction. 

2. True to time they come into view, and true to 
time they pass out, attracting and being attracted by 
one great, central, solar force, and also by other bodies, 
less or more, which may be near to them, or more re- 
mote. Through these contending influences LexelFs 
comet lost its balance, and wandered for a time among 
the moons of Jupiter ; but, by the natural laws of com- 



QuESTiONS. — What of the second ? 4. What of the third ? The 
nuclei of how many are known to be planetary in form? What is 
said of the nuclei of others ? 

Sec. LVIII. — 1. Have comets a real, material existence? How is 
that proved ? Have any of them returned according to prediction ? 
2. Do the great majority of comets revolve around the sun, like the 
planets ? Are comets more liable to be drawn from their orbits than 
other bodies ? Whv ? 



RARITY OF COMETS. 145 

pensation, it was let free, that it might fulfil the endless 
task assigned it. 

3. Yielding as it did to the greater attractive force, it 
proved itself material, as others have done, by the curves 
which they describe when sweeping round the sun. 



SECTION LIX. 



^xaritjT of Comets. 

1. It being; now evident that the comets are material, 
we may inquire concerning the character of their com- 
position and nature of their elements. Are they such 
as compose the crust of the earth, or the atmosphere by 
w^hich it is surrounded ? Are they similar to those that 
enter into the composition of the animal or vegetable 
kingdom? Would they be precious if we had them, 
or worthless ? Are thev dense, or are they rare ? (See 
Fig. 38.) 

2. Concerning some of these questions we may theo- 
rize and conjecture, but concerning others we may 
arrive at more definite and intelligent conclusions. 
Judging from their magnitudes, we might infer that 
they were very massive, — even thousands of times larger 
than the earth. The diameters of some of them are 
over one hundred thousand miles ; and then we do not 
include the luminous train, which is sometimes over one 
hundred millions of miles in length. 

3. Were these bodies dense in proportion to their 
bulk, what would be the force of such flying worlds ? 
What would their erratic wanderings produce, in the 



Questions. — 3. Do they always yield to the greater attractive 
force? What does that prove? 

Sec. LIX.^ — 1. What are some of the inquiries made concerning 
comets ? 2. Might we infer that comets are very massive ? What 
is the diameter of some of them ? How long is the tail of some of 
them? 3. If their density was in proportion to their magnitude, 
what might thev do ? 

13 



146 



EARITY OF COMETS. 



economy of nature? are questions on which we dare 
scarcely venture a conjecture. They might unhinge 
all worldsj and seal forever the destiny of the universe. 
But this is not to be anticipated. 

4. Though comets occupy the largest space, they have 




Fig. 38. 

the smallest masses, of all the cosmical bodies that are 
known to exist. The amount of matter which they 
contain is almost nothing, when compared with what is 
known to be in other heavenly bodies. If calculations 



are correct, their mean 



average weight would fall even 



below the one-hundred-thousandth part of that of our 
earth ; that is, if one hundred thousand of them were 



Questions. — 4. Is less matter in any one of them than is known 
to exist in any other heavenly body ? How much heavier is the 
earth than one of them ? 



RARITY OF COMETS. 147 

collected into one, it would weigh still less than tlie 
earth on which we dwell. 

5. This fact is rendered even more conclusive when 
we view them through the telescope, and also in con- 
nection with the light of fixed stars when it passes 
through them. They appear, even in their denser parts, 
to be composed of transparent, vaporous matter, sus- 
pended in an infinitely rarer atmosphere, which extends 
sometimes thousands of miles in every direction. 

6. This atmosphere contains the nuclei of these 
bodies, and sometimes intermingles with tlie apparent 
cloudy matter that is collected near their centres. Gene- 
rally this apparent cloudy matter decreases in density, 
going from the nucleus till it becomes invisible near the 
outer limits of the body. The tenuity of these outer 
parts, and the great distance to which they extend, may 
be accounted for by the small amount of gravitating 
force exerted upon them. This force is in proportion to 
the size and density of the nuclei ; and as they are ex- 
ceedingly rare, there is not sufficient attractive force to 
draw their atmospheres into a small compass. 

7. If the earth was a shell, with the interior parts 
removed, our atmosphere would expand ; and if it con- 
tained more matter than it does, the atmosphere would 
condense into smaller bounds, by the increased force of 
gravity at the centre. This principle is also manifested 
by comets in the formation of their tails. Their lighter 
parts are driven off, when in the vicinity of the sun, by 
some exterior or interior influence, and they increase 
in size, the particles which compose them being so far 
away from their central attractive force. 



Questions. — 5. How is this known? How do the stars appear 
as they pass between us and them ? How do they appear in their 
denser parts? 6. Do they appear to get rarer in their outer parts? 
How is the rarity of their outer parts accounted for? 7. AVould 
our atmosphere expand if the eartli was only a shell ? What of the 
atmosphere if it contained more matter than it does ? Is it probable 
that the tails of comets are formed out of their bodies ? In what 
way? 



148 NATURE OF COMETS. 

8. This exemplifies the exceeding subtleness of the 
substance of which these bodies are composed, and may 
account in part for the wonderful developments of some 
of their elements, which we are sometimes permitted to 
behold. 



SECTION LX. 

Mature of Comets. 

1. Feom investigations which have been made, it is 
evident that rarity is not the only peculiar characteristic 
of comets. They appear to be neither gaseous nor 
fluid ; or, if they are, their substances are infinitely 
refined, as is evident from the principle of refraction. 
Terrestrial objects which are transparent, as comets are, 
change the direction of the rays of light when they 
pass through them, and the denser the transparent 
medium the greater is its refractive power. 

2. This is invariably true with all transparent, solid, 
fluid, or gaseous substances with which we are ac- 
quainted ; but this principle appears to fail when light 
passes through the bodies of comets. It does not mani- 
fest the least refrangibility ; from which we would infer 
that the elements of which they are composed are not 
analogous to any of those with which we are familiar. 

3. Atmosphere a thousand times rarer than what we 
breathe has the power of producing a refraction which 
is perceptible ; and if comets were composed of any thing 
similar to it, they would produce the same effect on this 
agent when it penetrates their bodies. But such phe- 
nomena are never manifested by them : therefore we 

QuESTiOiSrs. — 8. What does this change of form prove ? 

Sec. LX. — 1. Do comets appear to be either fluid or liquid? Are 
they transparent? What is said of the refraction of terrestrial 
bodies ? 2. Does a dense transparent body refract light most ? Do 
comets refract light as it passes through them ? What would this 
show ? 3. Must they differ in their elements from any of those on 
the earth, or be more than a thousand times rarer than our atmos- 
phere ? Why must this be so ? 



COMETS SHINE BY REFLECTED LIGHT. 149 

would infer that the elements that compose them must 
differ in their nature from any of the elements of the 
earth or atmosphere. 

4. This being the case, what can they be? What is 
the nature of their elements? Perhaps they are pri- 
mordial/ dust-like particles held in equilibrio^ by their 
native repulsive power, and the mutual attractive influ- 
ence which they have for each other. This appears to 
be the character of their composition ; but, if it is not, 
science must await her appointed time to solve the 
mystery, and be satisfied to converse only about such 
properties and qualities as are known to belong to them. 



SECTION LXI. 

Comets §biiu bg Jkflccteb J^igbt. 

1. Comets are opaque,^ or at least as much so as other 
cosmical* bodies that are recognized as non-luminous, 
and would never be visible were it not for the solar 
light which they reflect. In like manner is it with the 
moon and satellites ; neither would the planets be visible 
were it not that they reflect to us the light which is 
shed upon them by the sun. We incidentally alluded 
to this fact before, but did not fully explain the method 
whereby we may be convinced of its truth. 



1 Primordial — existing from the beginning. 

^ Equilibrio — -held by two forces that are equal. 

^ Opaque — without light. 

* Cosmical — pertaining to heavenly bodies. 



QuESTioxs. — 4. Of what, then, are comets composed ? Have par- 
ticles of matter, in general, two forces operating on them as particles? 
What must be the effect of either force on particles of matter when 
they are in equilibrio ? Is it necessary that they touch each other 
to be in equilibrio? 

Sec. LXI. — 1. Are comets opaque bodies? Are the moon, satel- 
lites, and planets also opaque? How can we see them, if they are 
so? Is the sun self-luminous? 

13* 



150 



COMETS SHINE BY REFLECTED LIGHT. 



2. It has been discovered that light exists in two 
states, — natural and polarized. The former is invariably 
emitted from a gaseous, ethereal substance when heated 
to a certain temperature, and the latter from a liquid 
or solid substance when in the same state. But this 
inquiry has been carried further, and it has been ascer- 
tained that all reflected light, whether thrown off by 
rare or solid substances, is polarized. Now, this is tlie 
kind of light that we receive from all the bodies that 
are known to revolve around the sun, comets not 
excepted. Hence we may infer that they are opaque, 
and never would be visible were it not that thev reflect 




Fig. 39. 



to us the rays of light that fall upon them from the 
(See Fig. 39.) 



sun 



Questions. — 2. How many states do we find light in? What 
kind of substance emits natural light? What kind polarized? Is 
all reflected light polarized? Is this the kind that we receive from 
the comets? What does this prove? Would they ever be visible, 
without sunlicrht ? 



COMETS SHINE BY REFLECTED LIGHT. 151 

u. But tliis fact may be further exemplified by an 
establislied property which is common to all self-lumi- 
nous bodies that have a sensible disc. If the sun were 
seen from the outermost planet, he would seem as bright 
as if beheld by us ; and the only difference tliat we 
could observe would be in his size. True, the amount or 
volume of light would not be the same at these unequal 
distances : still, what would be received by observing 
him from Neptune would appear as bright as what we 
are, day by day, permitted to enjoy. 

4. This principle holds in regard to all self-luminous 
bodies that have an angular magnitude, but fails when 
applied to those that si line by borrowed light. They 
decrease in brightness the ftirther they are removed 
from the source of light, till they become invisible for 
want of a sufficient amount of reflected light whereby 
they may be seen. This is an accredited result, obtained 
by actual experiment, and is conclusive in proving the 
opacity of the comets. 

5. Their light increases and diminishes in brightness 
as they approach or recede from the source of light. 
This is obvious in the most brilliant, as well as in 
those that are scarcely visible. They reflect most when 
nearest to the sun, and least when at the greatest dis- 
tance ; which confirms the fact that they receive their 
light from him. 

Questions. — 3. Are all self-luminous heavenly bodies that have 
sensible discs equally brilliant? Wherein do they differ in relation 
to their light? 4. Is this a known principle in relation to all self- 
luminous bodies that haye an angular magnitude ? How is it when 
applied to bodies that shine by borrowed light? Do their rays 
diminish in brilliancy as they recede from us? What does this 
proye in relation to comets? 5. Does the brilliancy of their light 
increase and diminish as they approach and recede from the sun ? 
Is this the case with all the comets that are visible ? 



152 ANO:\[ALOUS ASPECTS OF COMETS. 



SECTION LXII. 

^nomnlous ^sprcts of diomds. 

1. NoTTTiTHSTANDiNG it IS truG that the denser parts 
of comets, which are the most brilliant, decrease in size 
as they approach the sun and expand as they recede 
from him, still, they are not outside of every analogy in 
their manifestations. These phenomena apparently con- 
tradict the general laws of heat, whereby objects increase 
in volume as they increase in temperature. This is one 
of its fundamental laws ; and its expansive effects may 
be observed by applying it to any substance that we 
may select. They all expand under its influence, except 
tlie denser parts of comets, which appear to contract as 
they approach the source of heat. 

2. To decipher this anomaly,^ many theories have 
been invented and many speculations suggested. Pro- 
fessor Encke attempted to explain it by assuming the 
existence of an ethereal fluid which pervades all space 
and grows denser in the vicinity of the sun." As the 
comet travels into these denser regions, it would be 
under greater pressure, which would reduce it in its 
volume. This might be true if it was elastic, and im- 
pervious to the medium through which it moved; but 
this is entirely unknown. 

3. In like manner did others attempt to solve the 
query by assuming an inherent power of condensation 
and expansion in the comets themselves ; but this was 
also objectionable ; for the change of volume always 
corresponds to their change of distance from the sun. 
The nearer they approach the source of heat, tlicir 

1 Anomalous — contrary to rule. 



Questions. — 1. What is anomalous in the appearance of the heads 
of comets? Do objects generally increase in size as thev increase in 
temperature? How is it with the heads of comets? 2. How did 
Profisssor Encke explain it ? 3. How did others attempt to solve tlie 
problem ? 



ANOMALOUS ASPECTS OF COMETS. 153 

nuclei grow less, and at the greater distance tliey expand. 
Though this is anomalous, yet it corresponds to some 
things on the earth with which we are familiar. 

4. Aqueous vapor, which is invisible, may be con- 
densed by cold into a visible fog, and dissipated again 
by the application of heat. Heat renders it wholly in- 
visible, and cold converts it into a visible object. May 
not a process somewhat analogous to this be carried on 
in the comets by the unequal degrees of heat and cold 
which they experience ? May not the nebulous matter 
which reflects the light be dissipated, when in the 
vicinity of the sun, into an invisible gas or vapor, 
thereby diminishing the bulk of reflective matter in 
their nuclei ? And may this invisible matter not gra- 
dually condense by attraction as they recede from the 
sun, thereby increasing the nebulosity as rapidly as it 
was diminished ? 

5. It is probable that this is a true explanation of 
these anomalous changes ; but there is another change 
observable, which is constantly in one direction. They 
are all becoming smaller at every revolution, diminish- 
ing both in bulk and brilliancy. Halley's comet, and 
others ^vhose histories are known for hundreds of years, 
are grooving less, and ere long some of them will be 
lost even to telescopic view. This fact is fully con- 
firmed ; and it now addresses itself to us for a truthful 
and rational solution. 

6. If the tails of comets emanate from their bodies, 
this proposition may not be so difiicult to solve. We 
are all familiar with the fact that some of them are of 
enormous length, extending sometimes more than one 
hundred and fifty millions of miles from the nuclei. 

Questions. — 4. Will heat dissipate aqueous vapor? "Will cold 
condense it ? Is it visible when dissipated ? Is it when condensed ? 
Might the sun's heat on the heads of comets cause them to appear 
less as they approach him? Might cold cause them to appear 
larger as they recede from him ? 5. What other change are comets 
undergoing? What is said of Halley's comet? 6. What is the 
length of the tails of some of the comets? Is it probable that por- 
tions of their tails are detached from their bodies and lost in space? 



154 TAILS OF COMETS — HOW FORMED. 

This being the case, the outer part of the tail may be 
thrown beyond the attractive power of the body to 
bring it back again by some exterior or interior influ- 
ence, and it may be irretrievably lost in the infinitude 
of space. 



SECTION LXIII. 

dls of Conuts — Jobj J^ormeb. 

1. The inquiry may naturally arise, What power urges 
the particles of matter, of which the tails of comets are 
composed, so far away from the nuclei ? or are the tails 
at all material ? To this inquiry we may respond more 
in the language of theory and probability than in the 
language of absolute truth and certainty. 

2. Some have supposed them to be produced by the rays 
of the sun passing through the nucleus, which is trans- 
parent and operates as a lens. Others thought that they 
were produced by the atmospheres of the comets being 
driven backward from the sun by an impulse of his rays. 

3. Sir Isaac Newton imagined that they were a thin 
vapor rising from the heated nucleus, as smoke ascends 
from the earth ; while Dr. Hamilton supposed that they 
were streams of electricity excited and dispersed by the 
influence of solar heat. (See Fig. 40.) 

4. Whatever these conjectures may be worth, they 
have not satisfied the inquiring mind ; neither can any 
one of them be maintained by any principle in science 
or natural phenomena, as they have been presented. It 
may be that the exterior parts of comets, which are 
almost spiritual in their nature, are rarefied when near 
the sun, and fall behind the more solid parts as they speed 



Questions, — 1. Are- there different theories in relation to the 
formation of the tails of comets? 2. What have some snpposed 
them to be? Wliat have others? 3. What was Sir Isaac Newton's 
theory? What Dr. Hamilton's? 4. May it be possible that they 
have an inherent power in themselves excited by the heat of the sun 
as they approach him, whereby they are formed ? 



TAILS OF COMETS — HOW FOUND. 



155 




Pio-. 40. 



156 TAILS OF COJSfETS — HOW FORMED. 

their way through space. Or it may be that there is an 
inherent power in the comets themselves, excited by the 
solar heat ; or a repulsive power in the sun, which 
drives from their bodies the lighter parts, both lumi- 
nous and transparent. 

5. But the inquiry may arise. What can this force or 
power be which counteracts the force of gravity ? Is 
it material, or immaterial ? Is it electrical, or magnetic ? 
or is it produced by a combination of them both? 
Probably it is the result of their united influence, as 
they are mutually productive of each other. 

6. By disturbing the magnetic fluid, electricity mani- 
fests itself; and by causing a current of electricity to 
flow through a conductor, magnetism is observed to 
accompany it. These fluids appear to be productive of 
each other ; and this circumstance may account for the 
polarity of the earth. 

7. Electricity will flee from heat, or heat will cause 
the fluid to circulate or pass from one point to another; 
and if electricity circulates around a piece of iron, it 
will become magnetic. Let us apply this principle to ex- 
plain the magnetic phenomena of the earth. The earth 
is the great reservoir of electricity, and is constantly, by 
its daily motion, turning its cold side to the sun, which 
creates currents of electricity at its equator, by whicli 
the magnetic poles of the earth are produced. 

8. But, besides the mutual influence of these fluids, 
they are possessed of antagonistic forces. By a simple 
change in the arrangement of the plates of a galvanic 
battery, its positive pole becomes negative, and its nega- 
tive becomes positive. So also is it with the magnet. 



Questions. — 5. What can this inherent power be which in a mea- 
sure counteracts the law of gravity? Is it electric, or magnetic? 
6. If the magnetic fluid is disturbed, does electricity manifest itself? 
If a current of electricity is created, does magnetism always accom- 
pany it? 7. From these circumstances may we account for the mag- 
netic poles of the earth? By what method ? Does nuignetism mani- 
fest itself at right angles to a current of electricity ? 8. Are magnet- 
ism and electricity possessed of repulsive forces? 



NOTIONS CONCERNING COMETS. 157 

By reversing its poles in relation to the circulating 
current of electricity, they assume opposite magnetic 
conditions. 

9. Now, it may be that there is an opposite force of 
polarity between the sun and the comets (which being 
increased by the heat of the sun), that produces their 
tails. Or it may be that as they approach him his heat 
creates opposing electro-magnetic influences in the comets 
themselves, which are productive sometimes of a plu- 
rality of tails. 

10. Their particles may be in different electric or mag- 
netic conditions, and may be stimulated to separate by 
an increase of temperature as they approach the sun. 
Polarity, electric or magnetic, conditioned by heat, may 
account for the origin, expansion, and contraction of 
these mysterious appendages, which have so often terri- 
fied not only the ignorant, but many also who profess 
to believe in the unbounded wisdom of an infinite 
Architect. 



SECTION LXIV. 

Superstitious ^[otioits tonanung €omtt$. 

1. Fear reigns in the hearts of all, especially when 
any thing extraordinary occurs. Spectres often create 
it, and apparitions often increase it, till it manifests itself 
in the wildest enthusiasm or the most nonsensical devo- 
tion. This has been the case at the advent of comets. 

2. Previous to the time when it was discovered that 
they obey the law of gravitation, they were viewed as 
the harbingers of evil, and sometimes as the avengers of 



Questions. — 9. How is the polarity of a magnet changed from 
positive to negative, and vice versa? 10. Might magnetic or elec- 
tric repulsion between the particles of the comets, or between them 
and the smi, conditioned by heat, produce their tails? 

Sec. LXIV. — 1. How were comets viewed by the superstitious 
during their early history ? 2. Were they ever viewed as the har- 
bingers of evil ? 



158 NOTIONS CONCERNING COMETS. 

wrong. Professor Wbiston charged one with avenging 
the iniquities of the antediluvians by producing the 
flood, and with carving up the surface of the earth with 
its tail, so that it now appears with its heights and liol- 
lows, like a prodigy of misery. 

3. Some charged them with being the precursors of 
famine, war, and pestilence, while others viewed them 
as disembodied spirits clothed with fiery indignation, 
or the final abode itself of such as are consigned to the 
never-ending torments of the Divine displeasure. 

4. The comet which made its appearance shortly 
before the Christian era, was supposed to be the meta- 
morphosed soul of Csesar, armed with fiery vengeance, 
that he might avenge his enemies ; but it continued its 
journey without using any artillery, or manifesting the 
slightest indications of war. 

5. So also was the comet of 1402 supposed to be omi- 
nous of unutterable evil. When it became visible, the 
Turks were fast extending their victorious arms over 
Europe, while Satan himself was supposed to be urging 
them on and making personal attacks upon the Church. 
A general gloom ensued, and there were fearful appre- 
hensions of a fast-approaching doom. 

6. Under these impressions, the people seemed totally 
regardless of the present, and anxious only for the 
future. To prepare for this appalling crisis and expected 
final doom, both the Pagan and Christian world became 
full of reverence and devotion. 

7. The miser himself became extravagantly benevo- 
lent, and the hypocrite tried to become sincere. Those 
who never feared before, sought to be rescued from the 
impending danger, and petitions were offered every day 



Questions. — With what did Professor Whiston charge one? 3. 
Were they ever charged witli being the precursors of famine, war, or 
pestilence, or with being the abode of condemned spirits ? 4. What is 
said of the comet that appeared shortly before the Christian era? 
5. What is said of the comet of 1402? What impression did it 
make upon the people? 6. How did they act? 7. What is said of 
the miser and the hypocrite ? 



])1STANCES OF FIXED STAES. 159 

for protection from the calamity almost at hand. " Save 
uSj oh, save us !" was uttered in loud acclaim by every 
land ; and as some appeared to have a more vivid con- 
sciousness of the causes of evil than others, they became 
more fervent, and prayed to be delivered not only from 
common dangers, but also from the power of the Turk, 
the devil, and the comet. 

8. But these fanatical reveries and superstitious devo- 
tions did not close the chimerical drama. Comets in 
general were supposed by some to be the prison-house 
of Satan and all his confederates, who were alternately 
burned and frozen by approaching and receding from 
the sun. And, as evidence of this, many imagined that 
they could see, in the darker parts of their tails, the 
struggling of fiends and the dingy clothes of the devil. 
But, in conclusion on this subject, it is evident that dis- 
covery has dissipated these fantasies, and corroborates 
the sacred testimony concerning the heavens and all 
their host, that the hand that made them is divine, that 
the wisdom that directs them is infinite, and that the 
power that controls them is omnipotent. 



SECTION LXV. 

JistHuxes of ^ktii ^tars. 

1. We propose now to take leave of the planetary 
system, all of the members of which are comparatively 
near to us, and extend our thoughts to the contem- 
plation of the fixed stars, which are far beyond its limits. 
When we pass outside of the solar system, we arrive at 
a tremendous vacuity, the general depth of which is at 
least sixty millions of millions of miles in every direc- 

QuESTiONS. — What is said of others ? From what did they pray 
to be delivered? 8. What were comets in general supposed to be? 
What evidence was adduced in favor of this belief? 

Sec. LXV. — 1. Are the fixed stars near the planetary system? 
How far distant mav we reckon some of those that are nearest to us ? 



160 DISTANCES OF FIXED STARS. 

tion. It is a great open space, destitute of glittering 
stars and glowing worlds to animate the solitude or take 
part in the economy of the universe. This is evident 
from a number of considerations. 

2. All material bodies attract each other with forces 
equal to their masses and the distances that they are 
apart. If the fixed stars were near to our solar system, 
some of the outermost planets at least would be sensibly 
affected by their attractive influence ; but no such result 
has ever been observed. Much more so would it be 
with the periodic comets which travel far beyond the 
utmost verge of the planetary system. They would be 
detained in their journey, and would not return accord- 
ing to prediction, as we find they do. 

3. Neither would the stars themselves appear to keep 
their relative places, were it not for the enormity of 
their distances from the earth. The earth, in making 
her annual revolution around the sun, changes her 
place in every six months one hundred and ninety mil- 
lions of miles : still, it effects no sensible change in their 
relative positions. From these circumstances, and evi- 
dence noticed elsewhere of the modes of approximating 
at stellar distances, we may infer that our planetary 
system is at an almost inconceivable distance from the 
starry heavens wn'th which it is surrounded. 

4. Passing beyond this abysmal void, we reach the 
nearest of the fixed stars, or those heavenly bodies 
which are called fixed, because they do not appear to 
change their places. They always manifest the same 
relative positions, and, in consequence of their apparent 
magnitudes and juxtaposition of some of them, they 



Questions. — What occupies the space between them and us ? 2. 
What evidence have we that the fixed stars are not near the planet- 
ary system ? 3. Do the stars appear to change their places with 
each other? What does that prove? What distance does the earth 
change in six months? Does this change of distance make any 
sensible change in the relative positions of the stars? AVhat con- 
clusion can we safely arrive at from the foregoing evidence ? 4. Why 
are the stars called fixed ? 



CLASSIFICATION OF FIXED STARS. 161 

have been classified and gronped into clusters, wliich 
clusters are named according to fhe fancies of their early 
observers. By these imaginary divisions and classifi- 
cation they are known when we wish to describe them 
or make them the special objects of investigation. 

5. In this way, if we are familiar with what is termed 
by some the geography of the heavens, we can as 
readily know the relative place of any star or constella- 
tion as we can determine any natural division on tlie 
earth. AVith these facilities to direct us, and the tele- 
scope to aid us, we will now enter into the boundless 
domain of the stellar universe, to contemplate the nature 
and number of its shining inhabitants, also their physical 
and optical relations, and the laws by which they are 
governed. 

SECTION LXVI. 

Classification of ^ktii ^tars. 

1. The stars, in contradistinction to planets, satellites, 
and comets, which ai'e visible to the naked eye, are 
arranged into six classes, according to their different 
degrees of brightness ; and those that are visible only 
by aided vision are divided by some into ten classes, in 
a similar manner; making, in all, sixteen divisions. 
(See Fig. 41.) 

2. The first class embraces all of the largest stars 
that are of the same brightness ; and they are about 
twenty in number. The second class, or those of the 
second degree of brightness, contains about sixty ; the 

QuESTioxs. — Have they been classified, and the heavens divided 
into districts by imaginary lines? Were certain names given to the 
clusters of stars that are in these divisions ? 5. Are these divisions 
readily distinguished by their names and appearance? How have 
we obtained the principal part of the knoAvledge which we have of 
the fixed stars ? 

Sec. LXVI. — 1. How many classes of stars are visible to the naked 
eye? How many classes visible only by aided vision? 2, What 
does the first class embrace ? What the second ? 



162 CLASSIFICATION OF FIXED STAES. 

third, one hundred and ninety ; the fourth, four hundred 
and twenty-five ; the fifth, eleven hundred ; the sixth, 
four thousand ; the seventh, twenty-six thousand ; the 
eighth, one hundred and eighty thousand; the ninth, 

STARS OF DIFFERENT MAGNITUDES. 

2 3 4 5 6 




11 12 13 14 15 

Fig. 41. 

eleven hundred thousand; the tenth, seven millions; 
the eleventh, fifty millions ; the twelfth, three hundred 
millions ; the thirteenth, five hundred millions ; and the 
smaller ones appear to be innumerable. 

3. From this table it may be observed that as the 
stars decrease in magnitude they increase rapidly in 
number, and more than nineteen -twentieths out of the 
many millions that belong to our astral system are 
under the tenth magnitude. This decrease of magni- 
tude and increase of number is what we might antici- 
pate when we consider our position in relation to them. 

4. We are surrounded on all sides by the stars ; and 
if we were to draw a circle touching those that are 
nearest to us, and one at double the distance from us, 



Questions.— What the third? What the fourth? What the 
fifth? What the sixth? What the seventh? What the eighth? 
What the ninth? What the tenth? What the eleventh? What 
the twelfth ? What the thirteenth ? What is said of the number of 
smaller ones ? 3. Do the stars increase in number as they decrease 
in magnitude? What proportion is under the tenth magnitude? 
How is this accounted for ? 4. If the stars were equally distributed 
in space, and a series of circles drawn among the stars, exterior and 
equidistant from each other, near which circle would be the greatest 
number of stars ? 



CLASSIFICATION OF FIXED STARS. 163 

fewer stars would be found on the nearer circle than on 
the one more remote, even if they were equally dis- 
tributed in space. This would be true no matter how 
many circles should be drawn at equal distances apart 
exterior to each other. At each successive exterior 
circle the magnitudes of the stars would decrease and 
their number would increase, even to the outermost 
limits of the stellar system, — if it has any limits. By 




Fig. 42. 

referring to Fig. 42, these facts may be readily under- 
stood. 

5. This universal classification of astral bodies as 
noted above is not based on their actual magnitudes, but 

Questions. — 5. On what is the above classification of stars based ? 



164 STARS HAVE NO SENSIBLE DISCS. 

on their different degrees of brightness, and these 
degrees of brightness are supposed to be in proportion 
to the different distances that they are away from the 
point at which we observe them. 



SECTION LXVII. 

^tars Ijab ito ^^nsiWe giscs. 

1. Stars do not appear lil^e planets when seen 
through the telescope. Planets present distinct, circular 
discs, similar to that which the moon offers to the naked 
eye, and their light appears to be in a measure diluted. 
With stars it is different. Telescopes with a magnifying 
power of six thousand, instead of increasing their size, 
appear to diminish even the largest of them to a mere 
brilliant point, having no sensible magnitude at all. 

2. This result is not because the lenses of the tele- 
scope do not magnify, but because the distances of these 
objects are so enormous, compared with their actual 
magnitudes, that their magnitudes at such great distanc m 
are more than six thousand times less than that which 
the naked eye is capable of perceiving ; that is, with all 
the facilities which have been employed for observing 
them, they appear to have no real magnitude at all. 
Glasses with low magnifying powers produce stellar 
discs ; but they are spurious, and are entirely dissipated 
by those of higher power. 

3. Their occultations by the moon manifest also that 
they have no sensible discs. When she intervenes 
between us and a planet, the light of the planet is gra- 
dually obscured ; but this is not • so when she passes 
between us and the stars. They preserve all of their 

Questions.-— What do we infer in relation to the distance of a 
star from its brilliancy ? 

Sec. LXVII. — 1. What effect has the telescope on planets ? What 
on the stars ? 2. Why do the stars appear so ? 3. What do their 
occultations manifest ? 



EEAL MAGNITUDES OF STARS. 165 

lustre until the inoincnt they come in contact with the 
dark edge of the moon, and then their light is instantly 
extinguished, \vithout the slightest appearance of a gra- 
dual diminution of brightness. 

4. By this sudden transition, and other facts already 
noticed, we discover that their masses have to us no per- 
ceptible magnitudes, notwithstanding they have been 
made the subjects of the most rigid scrutiny. 



SECTION LXVIII. 

Ileal Paqmtubes of ^tars. 



1. Since the stars present to us nothing except mere 
points of light, is it true that they are nothing else, 
and have no substantial magnitudes at all ? This ques- 
tion propounded itself long since, and was satisflictorily 
answered by Dr. Wollaston and others, who instituted 
a series of observations Avhich terminated in an estimate 
of the brio'htness and mao-nitude of some of the fixed 
stars in comparison with the sun. 

2. To aid them in the accomplishment of their designs, 
they brought into requisition the Photometer, an optical 
instrument the use of which is to ascertain the com- 
parative brightness of luminous objects. By this instru- 
ment the numerical ratio of two lights emitted from 
shining bodies is determined, whether near at hand or 
afar off. 

3. Prosecuting these investigations in relation to 
Sirius (the Dog-star) and the sun, it was discovered 
that the light received by us from the star was twenty 



Questions. — In what manner? 4. Would it be reasonable to 
infer that the stars are nothing but lucid points, ^Yithout further 
evidence? 

Sec. LXVIII. — 1. Who undettook to prove that the stars are 
large, massive bodies ? 2. What optical instrument did they use ? 
What is its special use ? 3. Wliat tAvo heavenly objects did they 
select ? 



166 REAL MAGNITUDES OF STARS. 

billions of times less than that received from the sun. 
This fact being obtained, and knowing from fixed prin- 
ciples in science that the farther luminous bodies are re- 
moved from us the less will be their apparent magni- 
tudes and brightness, it may be readily determined how 
far the sun would have to be removed back in space till 
his light would be only equal to that of the star in 
question. 

4. Light decreases in intensity in an inverse ratio as 
it recedes from the luminous body ; for if the sun was 
removed to twice its present distance from us its light 
would appear four times less, at three times its present 
distance it would be nine times less, and at ten times its 
present distance it would be one hundredfold less ; and 
so on, in the same ratio. 

5. Having computed the actual diameter of the sun, 
and being assured by this conceded principle in optics 
of the rapidity with which light decreases passing out 
from a luminous object, it was discovered by a simple 
calculation in arithmetic that the sun would have to be 
removed to one hundred and fifty thousand times its 
present distance that it might appear to us as the Dog- 
star. 

6. Now, if this change of distance produces such 
results in relation to the sun, may it not be reasonably 
inferred that the Dog-star is greater in magnitude, since 
it has been determined that it is more, far more, than one 
hundred and fifty thousand times his distance from us ? 
In like manner is it with all the stars. They are suns. 



QuESTiojJS. — How much less is the light of the Dog-star than that 
of the sun ? Do luminous bodies appear to grow less the farther 
they are removed from us ? From known principles, could it be dis- 
covered how far the sun would have to be removed from us till he 
would appear equal in size to the Dog-star? 4, If the sun was re- 
moved to twice his present distance from us, how much would his 
light be diminished? Light is governed by what law, as the lumi- 
nous body changes its distance from us ? o. How far would the sun 
have to be removed before he would appear only the size of the Dog- 
star? 6. Ts the Dog-star more than one hundred and fifty thousand 
times the distance to the sun ? 



DISTANCES OF THE STARS. 167 

the majority of which are no doubt many times larger 
than our sun, but a}3parently less, in consequence of 
their great distance from us. 



SECTION LXIX. 

Jllttljob of dLomputtng distances to ^m'is ^lars bg ®^rigouometri|. 

1. From what has been said in the preceding section, 
the inquiry may arise, How is it known that the dis- 
tance to Sirius is more than one hundred and fifty thou- 
sand times the distance to the sun ? or, in other words. 
How can we determine the distance to any fixed star ? 
We determine the distances of the sun, moon, and 
planets from the earth, by the principles of plane trigo- 
nometry -^ and by the same principles do we determine 
the distances of some of the nearest stars. 

2. Suppose this curved line in Fig. 43 to represent 
the orbit of the earth, which is one hundred and ninety 
millions of miles in diameter, and S a fixed star, the 
distance of which we wish to determine. If we will 
take an observation of the star w^hen the earth is at one 
side of its orbit, and in six months afterwards also when 
it arrives at the opposite side, then we w^ill have a 
triangle, two lines of which are visual lines, and the 
third the diameter of the earth's orbit. Now, from the 
known elements of this triangle, or those that may be 
readily known, viz., the angles and one side, which is the 
diameter of the earth's orbit, those lines that are un- 



^ Trigonometry — that science which teaches how to determine the 
several parts of a triangle from having certain parts given. 



Questions. — What may we infer in relation to his size in relation 
to that of the sun ? 

Sec. LXIX. — 1. What mathematical principles are involved in find- 
ing the distances to the sun, moon, and planets? Can we determine 
the distances of some of the nearest stars on the same principles? 
2. What kind of a figure would we form in finding the distance to a 
star ? What would we use as a base-line ? 



168 DISTANCES OF THE STARS. 

known are easily determined, either of which would be 
the distance to the star in question. 

3. By this method the distance to some of the nearest 
fixed stars have been discovered ; and were it not that 




Fig. 43. 

the visual lines of observation appear to coalesce before 
reaching the object, the distances of at least some of 
those that are more remote might be known. If the 
diameter of the earth's orbit were increased, greater 
distances into space could be mathematically measured, 
and many heavenly bodies, now in point of distance 
outside of computation, would fall within its limits. 



SECTION LXX. 

f elestopic gletljob of Computing gtstanccs of p^abenlg WjttU. 

1. Assuming now, what may not be rigorously true, 
but what may not be very far from it, that the stars 
would appear equal in magnitude if they were at the 
same distance from us, we arrive at this astonishing re- 

QuESTiONS. — What elements in tlie triangle are known, or can 
easily be known ? 3. If the diameter of the earth's orbit was greater, 
could we measure the distances of objects more remote? 

Sec. LXX. — How long would it take light, travelling at its usual 
rate, to rev2h u? from the nearest fixed stars? 



DISTANCES OF THE STARS. 169 

suit, that it would take light ten years to reach us from 
those of the first magnitude, and one hundred and 
twenty years from those of the sixth magnitude, tra- 
velling at the rate of twelve millions of miles in a 
minute. But the inquiry may arise, Have we any know- 
ledge of the distances of telescopic stars, or those that 
are sunk so deep in space that they are invisible with 
unaided vision ? 

2. Sir William Herschel attempted to explore the 
unknown regions of the heavens with his gigantic tele- 
scope, and did not fail to reveal much that was hitherto 
unknown. That he might be able to accomplish his 
purpose, he first tested the space-penetrating power of 
his instrument by the power of the human eye. In 
order that we may see an object, it is necessary that a 
sufficient number of rays of light enter the pupil of the 
eye to make a sensible impression on the seat of vision. 
When this is not the case, the object will be invisible. 
In like manner is it with the telescope. If a sufficient 
amount of light sent forth from a body does not enter 
the object-glass so as to make an impression on the 
retina of the eye after it is concentrated by the lenses, 
the object will be invisible even with the use of the 
instrument. 

3. From this it may be observed that the telescope is 
constructed on the principles of the eye, and that by the 
one we are able to measure the power of the other. 
This may be illustrated by the use of objects with which 
we are familiar. 

4. If Ave should observe letters written on an object 
one mile distant, and be no more than able to read them, 
this would give us the measure of the power of our 



Questions. — How long from those of the sixth magnitude ? Can 
the distance to telescopic stars be determined ? 2. Who was the first 
great explorer of the heavens ? How is the power of the telescope 
tested ? What is necessary that an object may be visible by the eye 
or the telescope ? 3. On what principle is the telescope constructed ? 
Can the power of the telescope be ascertained? 4. How can we 
ascertain the power of the eye ? 

15 



170 DISTANCES OF THE STAES. 

eyes. To read the same letters twice the distance would 
require eyes increased in power to correspond with the 
increase of distance, or, in other w^ords, with pupils of 
larger apertures for the admission of more light. But, 
as the pupils cannot be enlarged, the object-glass of the 
telescope answers the same purpose as if they were ; and 
as its dimensions, in comparison with those of the pupil 
of the eye, can be readily determined, ^ve at once may 
ascertain how much farther it can penetrate into space. 

5. With this information gained, Herschel undertook 
to explore the heavens, and, if possible, to fathom the 
depths of that zone of light familiarly known as the 
Milky Way. Arranging the visible stars according to 
their mao^iitudes into six classes, and considerino^ the 
smallest to be at the greatest distance, he reached out 
still farther into the fathomless abyss of space, and 
descried other worlds glowing with light and beauty. 

6. Pursuing these investigations, many things won- 
derful passed in hasty review before him. Instead of a 
comparatively few brilliant points of light which we 
may witness in starlight nights, thousands of worlds 
moved over his field of view, and that arch of milky 
light which encircles the heavens resolved itself into 
innumerable stars glowing with their own inherent 
splendor. Its hazy appearance was dissipated before 
the great pupil of the telescopic eye, and he witnessed 
realms of solar light so far aw^ay that it could not reach 
us, travelling at its usual velocity of twelve millions of 
miles per minute, in less than five hundred years. 

7. Such are some of the results arrived at by these 
insti'uments of science and art ; yet there are others more 
astounding. The nebulae are at a still greater distance. 
Some of them are so far removed that the telescopes of 

QuEsriONS. — In the tolescop?, what represents the pnpil of tlie 
eye ? By knowing the power of the telescope, can we discover the 
distance "that we can see with it? 5. What portion of the heavens 
especially did Herscliel explore? 6. What did he witness wlien 
examining it? How long would it take the light of some of the 
objects that he saw, to reach us? 7. What other objects did he dis- 
cover? Did he resolve them by his telescope? 



THE STARS ARE SELF-LUMINOUS BODIES. 171 

the highest powers cannot perfectly resolve them ijito 
stars. This being so, and knowing the distance to 
which the telescope will penetrate, we may safely infer 
that 'they are so far away that their light would only 
reach us after a journey of thirty thousand years. 

8. But this is not alh There are other nebulae which 
manifest no signs of resolvability, though they may be 
telescopically scrutinized under the most favorable cir- 
cumstances. Immutable as they are under the most 
rigorous tests that science can employ, or art construct, 
what can be their distance from our earth ? From 
Herschel's computations, his great reflecting telescope 
would follow^ some of them if they were plunged so 
deep in space that their light would require fifty thou- 
sand years to reach us ; and the great telescope of Lord 
John Rosse would pursue them to ten times this incon- 
ceivable distance. 

9. Does not reason stagger at these results, and is the 
imagination not confounded? Yet these are only a 
part of nature's works ; and no one can know them to 
perfection, much less the infinite wisdom and powTr of 
Him who is the Builder of them all. 



SECTION LXXI. 

€1qz ^tnxs mz ^elf-ITuminous gobies. 

1. Having considered briefly the apparent magni- 
tudes and distances of the stars, both visible and tele- 
scopic, we will now notice their physical character, also 
their relations and their general distribution in space. 
They are self-luminous bodies, shining by their owm 

Questions. — How long would it take the light of some of them 
to reach us? 8. Are there nebulae in the heavens which have never 
been resolved? How long would it take the light of some of them 
to reach us ? 9. Have we a conception of the extent of the physical 
universe ? 

Sec. LXXI. — 1. Are the stars self-luminous ? How has this fact 
been discovered ? 



172 THE STARS ARE SELF-LUMINOUS BODIES. 

inherent light, similar to our great orb of day, to which 
we are indebted for so many comforts. This fact has 
been made apparent by photometrical observations, 
which have been made on light emitted and reflected 
from various objects, as well as other considerations. 

2. The light of the planet Venus, when the greatest 
amount of her enlightened surface is next to us, is con- 
sidered three thousand times more feeble than the light 
of the full moon ; and the light of the full moon is 
ascertained to be three hundred thousand times less than 
the light of the sun. From these results, it may be 
inferred how feeble the light must be which is reflected 
from bodies that are comparatively near us, when con- 
trasted with the light of the sun, which is at such a great 
distance from us. But it has been shown that the stars 
are many millions of times farther away than the sun : 
consequently, if they were not suns themselves, and 
were shining by reflected light, they could not be seen 
even if they were millions of times nearer to us than 
they are. 

3. All bodies, no matter what their sizes might be, if 
they were destitute of a light-generating principle, 
would disappear in darkness even before they would 
have travelled over more than an infinitesimal part of 
the distance to the nearest of the stars. They would 
fade away from our view, never again to smile upon the 
earth or reveal to man, through the medium of light, 
that they still retained an existence. (See Fig. 44.) 

4. But the stars, even to the sixth magnitude, are 
visible to the naked eye, and the heavens near those of 
the first magnitude, when viewed through a large tele- 
scope, exhibit the appearance of an eastern firmament 



QuESTioxs. — 2, How much more feeble is the liglit of Venus than 
that of the moon? How mncli more feeble is the light of the moon 
than that of the sun ? Are the stars millions of times farther away 
than the sun ? If they were not self-luminous, could they be seen at 
this great distance? 3, Would they be visible if they were millions 
of miles nearer to us, if they were not self-luminous? 4. How do 
some of the stars appear when viewed through a telescope ? 



THE STAES ARE SELF-LUMINOUS BODIES. 



173 



at the approach of sunrise. And when some of the 
most brilliant enter into the field of view, the eye is 
almost overpowered by their dazzling light, unless pro- 
tected by the interposition of some translucent substance. 




Fig. 44. 

5. These facts, together with the fact that their light 
is natural and not polarized, are sufficient of themselves 
to satisfy even the inquiring mind that they are suns, 
some of which are no doubt hundreds of times larger 
than our own, and far more glorious in their splendor. 
Did I say glorious in splendor ? Yes, glorious ; for, 
wherever we may happen to turn the great eye of the 
telescope, suns of every class and color greet us with 
their glory. 

Questions. — 5. "What other evidence is there that the stars are 
suns like our own ? Are the stars of different colors? 

15* 



174 COLOR OF FIXED STAES. 

SECTION LXXII. 

Color of fxuh ^tnrs. 

1. The fixed stars are of a great variety of colors, 
and all of these colors are either natural or comple- 
mentary. The red star is in contrast with the green, 
the yellow with the blue, the rose-colored with the 
sapphire, and the greenish-white with the orange, all 
blending their variegated hues into one common spec- 
trum, exhibiting, in certain instances, a scene equal, if 
not superior, in beauty, to any thing that we were ever 
permitted to behold. 

2. Some of them are white, or blue, or red, or green, 
or yellow, varying in color, no doubt, according to the 
nature of the substances in which their light is gene- 
rated or through which it is transmitted. This is evi- 
dent from the fact that when they are viewed singly 
they universally present the same colors, which would 
not be the case if any of them were produced by con- 
trast. The natural color of a star may be changed by 
the light of another star mingling with its own light. 

3. It is a well-known optical phenomenon that a 
faint white light appears green when a strong red light 
is brought near it, and that a white light becomes blue 
when the stronger surrounding light is yellow. This is 
frequently manifested by multiple stars, or those that 
are comparatively near together. If the light emitted 
by a red star commingles with a feebler light from a 
white star, the white star will appear green ; and if the 
light from a yellow star commingles with the light of 
a white star, the light of the white star will be blue. 
These new colors which are produced in this way are 
called complementary, as has been previously noticed, 

QuESTioxs.— 1. Are these stars of nearly every color? 2. What 
is the probable cause of their difference of color? Does the color 
of one star change the color of another ? 3. In what class of stars 
is this manifested? What are the new colors formed in this wav 
called? 



NEW TEMPORARY STARS. 175 

and vary in degree accordinoj to tlie relative quantities 
of light of different kinds that enter into their compo- 
sition. 

4. Consequently, some stars lose their natural colors 
and assume the complementary hue, not by any physi- 
cal change in their photospheres, but simply by the 
union of the variegated rays which unite from different 
sources. But notwithstanding we are often deceived 
as it regards the natural color of stars, owing to these 
complementary hues, still there are stars, apparently 
isolated, which are possessed of positive colors, some 
of them being as red as blood, and others as blue as 
indigo. Sirius, the Dog-star, is one of them, and is the 
only one of this class which is known to have undergone 
any permanent change in the color of its light. Not- 
withstanding many of those stars that are comj^lementary 
in color have lost in a measure their brilliant hues, 
yet it is the only one of its claims known to have changed 
from its original ruddy color to a white, which is the 
most common color of these self-luminous bodies. 



SECTION LXXIII. 

1. Sometimes new stars burst forth into view, 
and old ones are observed to go out in darkness. 
Within the last two thousand years about twenty stars, 
hitherto unknown, have made their appearance in various 
parts of the heavens, but more especially along the 
Milky Way. This class of stars generally come sud- 

QuESTiONS. — 4. What causes the change of color to take place? 
Have some stars positive colors? What colors do some of them pre- 
sent? Do any that have positive colors change in color? Wliat 
was the color of the Dog-star? What is its color now? 

Sec. LXXIII. — 1. Do new stars ever appear in the heavens and 
others disappear? How many new ones have made their appearance 
within the last two thousand years? What is said of the magnitude 
and brilliancv of this class of stars? 



176 NEW TEMPORARY STARS. 

denly into view^ increase rapidly in splendor, and are 
of different magnitudes when at their greatest brilliancy. 
A few days frequently, and sometimes only a few hours, 
are necessary for their full development. 

2. In a majority of cases they scintillate more than 
other stars, and occasionally change in color as they 
change in brightness. Some of them become very bril- 
liant, rivalling in splendor stars of the first magnitude 
as well as the larger planets, and are sometimes visible 
even at noonday. No motion or change of place is 
manifested by any of them ; but the majority of them 
soon begin to fade away, and are invisible in a few days, 
whilst others are diminishing in brilliancy for twenty 
years before they totally disappear. 

3. These stars go out in darkness, probably never 
again to be resuscitated, whereby the sense of vision 
may be awakened that we may be able again to behold 
them. In like manner is it with other stars whose advents 
are unknown. A few are known to have vanished from 
the heavens, and the places which they once occupied 
are now voids in the map of the starry vault. They 
are lost to our view, not likely by a destruction of their 
physical constitutions, or by annihilation, but probably 
by a cessation of an electrical process in their atmps- 
pheres which may be productive of their light. 

Questions. — 2. Do these stars scintillate more than other stars, 
and do they ever change in color ? Are any of them ever visible in 
the daytime? What is said of the disappearance of these stars? 
3. Do any of these stars ever reappear ? Have any of the old stars 
ever disappeared from the heavens ? What is the probable cause of 
their disappearance ? 



NEW PERMANENT AND PERIODICAL STARS. 177 



SECTION LXXIV. 

1. Another class of stars known to astronomers has 
come into vicAV which is apparently permanent. Only 
a few of these stars have been discovered ; yet they arc 
snfficient in number to confirm the fact that they 
actually exist. They are similar to other stars, and are 
of various magnitudes. As comparatively little is 
known concerning them, we will now refer to those in 
which there is a periodical variation in their light. 

2. They are generally of a reddish appearance when 
at their maximum magnitude, but change sometimes to 
a yellow, and then to a white, as they decrease in bright- 
ness; and the order is reversed as their light again 
increases. These are more numerous than those of any 
of the unusual classes Avhich we have noticed; and they 
vary in their periods from three to five hundred days, 
and, it may be, even hundreds of years. Some of them 
are regular in their variations, and others very irregular, 
— so much so that no law appears to be applicable to 
them. 

3. The star Algol, in the constellation Perseus, i& an 
instance of those of regular periods. It changes from 
a star of the second magnitude to one of the fourth in 
three hours and a half, and returns to the same magni- 
tude in the same length of time. Remaining stationary 
in magnitude for two days, it again goes through the 
same variations. 

4. Other stars appear to oscillate back and forth in 
magnitude, diminishing from one magnitude to another. 



Questions. — 1. Have stars made their appearance which are 
apparently permanent? Do thev resemble other stars? 2. What 
are variable stars ? What is their color when at their maximum 
magnitude? Do they change in color ? Are their periods of varia- 
tion of equal length ? Are they all regular in their variations ? 
3. What is the period of Algol ? 4. Are there other stars that are 
very irregular in their variations in magnitude? 



178 NEW PERMANENT AND PERIODICAL STARS. 

and again increasing, afterwards diminishing and be- 
coming totally invisible, and then rekindling and re- 
turning again to their original brightness, only to enter on 
a greater diversity of temporary phases. Stars of every 
degree of brightness that can be distinctly observed are 
subject to these fluctuations ; but they are confined more 
especially to those embraced between the sixth and 
ninth magnitudes. 

5. To account for these peculiarities has occupied the 
attention of men of science ever since their discovery. 
Some have conjectured one cause, and others another. 
The circulating of opaque bodies around them, as the 
planets revolve around the sun, partially obscuring them 
at every revolution, is one theory. Another is that 
these stars may revolve on their axes, and may have 
very large dark spots on one side, and when the side 
which has the spots on is turned towards us their light 
would be diminished. 

6. May it not be that their light is generated by 
electrical currents in their atmospheres, and may the 
variations of brilliancy not be accounted for by the 
increased or diminished activity of this invisible agent, 
which probably pervades all worlds in greater abundance 
than it does our own ? Discovery and experiment seem 
to direct us to these conclusions, and untiring research 
may yet unfold more fully not only the secrets of their 
nature, but also of their relations. 

Questions. — Stars of what magnitude are more especially subject 
to these variations ? 5. What are some of the theories concerning 
these fluctuations ? 6. What is the probable cause of these changes 
in magnitude? What seems to direct us to this conclusion? 



RELATIONS OF MULTIPLE STARS. 179 

SECTION LXXV. 

Ilclattons of pultiple Stars. 

1. Stars are frequently united optically as well as 
physically, notwithstanding the great distances which 
separate many of them ; and the number of both kinds 
which have been discovered amounts to between five 
and six thousand. The law of gravitation — the great 
])arental law of the universe — prevails wherever matter 
exists, and holds all worlds and firmaments and 
schemes and systems subservient to the will of their 
Creator. Heavenly bodies are not only grouped together 
in consequence of their apparent juxtaposition, but they 
are actually held together by this universal force. We 
see it operating in every part of the universe. 

2. Planets, satellites, and comets are all held in their 
orbits by this law; neither are the stars insensible to 
its sway, though apparently isolated, or companions of 
each other. Many of them are united in pairs, and 
double and triple pairs, by its omnipotent control, and 
constitute systems of themselves, wliich are truly won- 
derful in their arrangement. 



SECTION LXXVI. 

3tm$ ©pticallg ^nitcb, xfc. 

1. Stars are sometimes united optically, when they 
have no" immediate physical connection. In the former 
case they appear near together, not because they are 
actually so, but in consequence of their being nearly o i 



Questions. — 1. In what two ways are stars apparently united? 
How many tbousainl have been discovered ? Are the stars subject 
to tlie kiw of gravitation ? 2. Are the pLniet:^, satellites, and comets ? 
Are some of the stars united by it in pairs aid systems? 

Sec. LXXVI. — 1. What do you understai d by an optical union 
of stars ? 



180 RELATIONS OF MULTIPLE STARS. 

the same line of vision. As may be seen by referring 
to Fig. 45, No. 1, they appear to the naked eye as one 
star, in consequence of their light travelling in the same 
direction and commingling before it reaches the eye of 



Fig. 45. 

the observer ; but when viewed by the telescope they are 
resolved into a number of individual stars, varying often 
in size and color, as may be seen by referring to Fig. 45, 
No. 2. 

2. It is known by actual experiment that the further 
we recede from objects the nearer they appear to approach 
each other, and the further we are distant from the stars 
their connection appears more intimate, till a number 
of them separated by almost inconceivable distances, 
though nearly in the same line of vision, present the 
appearance of a single body. About five thousand of 
these multiple stars have been discovered in both hemi- 
spheres which have only an optical connection, whilst 
there are only about six hundred similar in appearance, 
the component parts of which are known to hold to 
each other a far more intimate relation. 



Questions. — How do stars appear to the naked eye when united 
ill this way? How when observed by the telescope? 2. Do bodies 
aj^pear to approach each other the farther they are removed from us? 
Is this true in relation to the stars ? How many thousand stars have 
been discovered to have an optical union ? 



STARS PHYSICALLY UNITED, &C. 181 

SECTION LXXVII. 

Stars ^IjnsicaKg Unitcb, ct'c. 

1. Stars physically double, like those which we have 
described, resemble single stars, and they would not be 
known as any thing else, were it not for the descrying 
power of the telescope. It reveals their plurality by 
causing them to a])pear to separate into parts, and testi- 
fies to the inflexible power of those dynamical laws 
by which they are all controlled. Under its almost 
omniscient gaze, stars single in appearance are resolved 
sometimes into various parts which have a mutual 
motion around each other. 

2. These component parts, taken together, are denomi- 
nated systems, varying in name according to the number 
that compose them. The binary systems embrace two 
stars, and are by far the most numerous. The ternary 
systems embrace three stars; and only one hundred and 
thirteen of these are known to exist. The quaternary 
embrace four stars ; and only nine of them have been 
discovered. There are two quintuple systems, and 
only one sextuple and one octuple system, as yet, dis- 
covered. 

3. As already noticed, stars apparently single are re- 
solved into these various systems by the use of the tele- 
scope ; and even some of the component parts of these 
systems might be separated still further, if we had better 
facilities for observing them. A view of all of these 
systems is represented by Fig. 46. 

4. Letter A represents a binary system. Only a 
single star is first observed ; but when viewed by the 

Questions. — 1. What do you understand by a physical union of 
stars ? How do they appear to the naked eye ? How when examined 
with the telescope ? 2. When the component parts are taken together, 
what are they called? A system composed of two stars is called 
what? What if composed of three stars? What if composed of 
four? What if composed of five? What if composed of six? 
What if composed of eight? 3, 4. Has each of these systems a 
centre e f gravity ? 

16 



182 



STAES PHYSICALLY UNITED, &C. 



telescope it separates into two, which parts are known 
sometimes to have a mutual revolution around each 
other. These parts vary sometimes in color, and they 
have a centre of gravity of their own. This centre of 
gravity is a point in space distant from each according 
to their respective masses ; and the curves w^hich they 



A 


A 


B 


c 




IS^P 




l^^fl 


^^ J 




F HB"""'""'"'"'I"""^ 


^^^li^: 


**c ^*' 


.... -/^** 


i 


■::c;v#f':!^^>^g^:1il 



fig, 46. 



describe in their periodic revolutions will bear to each 
other a similar proportion. If the mass of one is com- 
paratively large, the curve that it will describe will be 
proportionally less than the curve which is described by 
its less companion. 

5. The periods of their revolutions vary from thirty 
to a thousand years, as is eviden^t from the fact that 
some of them have actually completed their periods 
around each other, according to calculation, since their 
orbital motions were discovered. The same is also 

Questions.— May it be a point in space, or one of the stars ? 5. 
What are the lengths of some of their periods? Are the othei* 
systems noticed in this section similar to the binary system ? 



» CLUSTERS OF STARS. 183 

true of the triple, quadruple, quintuple, sextuple, and 
octuple systems, which are re[)resented bv the folh)wing 
letters in Fig. 46, B, C, D, E, F. 

6. In the first instance each of these objects presents 
the appearance of a single star, and by the use of the 
telescope one separates into two parts, another into 
three, another into four, another into five, another into 
six, and another into eight stars, all of which compo- 
nent parts of each system have an intimate physical 
relation. They separate more especially into pairs, 
indicating that the parts of each pair have a closer con- 
nection than the pairs themselves. Each pair some- 
times has its centre of gravity, and one member revolves 
around another; while all of the pairs revolve around a 
centre of gravity common to the whole system. 

7. The centre of gravity of such a system would be 
a point in space, unless one of the bodies in the system 
should be very large when compared with the mass of 
all the rest ; and then they would circulate around it, as 
the planets do around the sun. For the stability and 
perpetuity of these systems, it matters not under which 
of these conditions they are placed, as they are all sub- 
ject to the force of one universal law, whose prerogative 
it is to reign over every thing material. 



SECTIOTsT LXXVIII. 

ClusUrs of ^t;irs. 



1. Besides the objects which we have attempted to 
describe, numerous clusters, composed of hundreds of 

Questions. — 6. How do they appear to the naked eye ? How do 
they appear to separate w^hen viewed by the telescope ? Has each 
pair a centre of gravity ? Has the wliole system also a centre of 
gravity? Are they all in motion around these centres? 7. If the 
centre of gravity of such a system was a very massive body, com- 
paratively, how would the others revolve ? Are all of these systems 
subject to phvsical laws with which we are in a measure familiar? 

Sec. LXXVTII.— 1. What is a cluster of stars? 



184 CLUSTERS OF STARS. 

glowing worlds, have been brought to view by the aid 
of optical instruments even of moderately penetrating 
powers. A group in the constellation Taurus, called 
the Pleiades,. is one of them. Six or seven stars in it 
are visible to the naked eye, and about two hundred 
more may be seen with the aid of the telescope, appa- 
rently wedged in and projected on one another, as if 
there Avas no other space for them to occupy. 

2. In the constellation called Coma Berenicis, there 
is another such group, though not so densely crowded 
together, and composed of much larger stars. Also, in 
the constellation Cancer, there is a cluster of very small 
stars, called Prsesepe, which is sufficiently luminous to 
be seen at night in the absence of the moon, and may 
be resolved into separate stars, even with an ordinary 
spy-glass. Another such group is also found in the 
sword-handle of Perseus, composed of smaller stars, 
from which we would infer that it is much farther 
from us. 

3. The general outline of these clusters is oval or 
globular, and they all appear to be more thickly popu- 
lated or grow denser towards their centres. Nearly all 
of them are composed of stars of different magnitudes, 
and frequently of different colors, notwithstanding a 
few of them are uniform in these respects. From the 
two specimens represented in Fig. 47, ^ome knowledge 
may be obtained of their general appearance, and we 
be led to inquire what wonderful dynamical arrange- 
ments can prevail in these stupendous sidereal systems, 
whereby order and harmony are maintained. Surely 
their component parts must fl^^ with great velocity, and 
forces inconceivable must be exerted on each other to 
perpetuate their stability and existence. 



Questions. — Are they numerous in the heavens? What one is 
that in Taurus? How does it appear when viewed with the tele- 
scope ? 2. What other constellations have clusters in them ? 3, What 
is the general form of these clusters ? Are the stars in them frequently 
of different colors ? 



CLUSTERS OF STARS. 



185 



4. If they all are suns, as we have evidence to believe 
they are, and each having an annual motion around its 




Fiff. 47. 



neighbor, and surrounded by circling planets, inhabited 

Questions. — 4. Are the bodies that compose these clusters self- 
luminous ? Are they all iu motion ? May they have planets and 
satellites revolving around them ? 



186 NEBULA. 

probably with rational beings like our own, what must 
be the picture presented daily to their view ! A red 
sun gilds the chambers of the east, a white one lights 
up the morning, soon a blue ofie spreads out its mild 
and mellow light, next a green one rises up to view, 
and, to variegate still more the color of the day, a yellow 
one blends its radiant beams in the chromatic picture, 
the equal of which the limner's pencil never painted, 
nor of which even Raphael had ever a conception. 



SECTION LXXIX. 

1. ADVANCiNa farther into the unknown realms of 
the universe, we arrive at other cosmical objects, far 
more mysterious in their natures, and far more difficult 
to investigate. They are the nebulae, or those patches 
of hazy light which are scattered, without any apparent 
regularity, in various parts of the heavens. They may 
be observed in whatever way the telescope may be 
directed ; and they vary in form and brightness almost 
as much as the stars are known to vary in magnitude. 

2. About four thousand of them have been dis- 
covered since they first attracted the attention of those 
devoted to the science of astronomy and engaged in 
exploring the heavens. Out of these four thousand, the 
greater number of which are in the northern hemisphere, 
nearly four hundred have been resolved, by the aid of 
the telescope, into clusters of stars ; and no doubt many 
more of them might be resolved by increasing the facili- 
ties for observing them. 



Questions. — If they have, what variations would occur in the 
light that constitutes their day ? 

Sec. LXXIX. — 1. What are the nebulae? Are they irregularly 
distributed over the heavens ? 2. How many of them have been dis- 
covered? Are they more numerous in the northern or southern 
hemisphere? How many have been resolved into stars? 



SECTION OF THE MILKY WAY. 



187 




fig. 48. 



188 NEBULA. 

3. From the appearance of some of them we might 
infer that they are within the limits of our stellar 
system, and from the appearance of others we might as 
readily infer that they are at an inconceivable distance 
beyond it. No sufficient data has been discovered 
whereby we may determine this uncertain question, 
much less be able to discover the nature of their physi- 
cal constitution. 

4. Some of these nebulae have been regarded, by men 
of great scientific ability who were favorable to the 
theory of a progressive formation and development of 
stars or worlds, to be self-luminous matter, undergoing 
a constant but almost imperceptible condensation. 
Others, of equal ability, are of the opinion that they 
are all great astral systems, buried so deep in space that 
nothing of them is visible, even with the aid of the 
most powerful glasses, except the faint glimmerings of 
their almost exhausted light. 

5. In justification of the conclusions of the former, 
they contend that some of the nebulae are growing less, 
losing the irregularity of their outlines, and becoming 
more globular in form. Also, they refer to the fact 
that there is celestial star-dust, or vaporous material 
substances, which are manifested by the zodiacal light 
and meteoric showers and comets within the planetary 
system, and why may not material elements analogous 
in their nature exist in various parts of the sidereal 
universe? Why may not these gaseous, nebulous 
masses exist in space like other heavenly bodies, and 
why may they not condense, through time, by their own 
molecular attraction, into worlds, and finally shine out 
like stars in the celestial firmament? 

6. On the other hand, it is affirmed that scientific dis- 
coveries direct us to far more rational and accredited 



Questions, — 3. "What is said of the places they occupy ? 4. What 
is the opinion of some astronomers concerning them ? What of 
others? 5. What eyidence is adduced by the former in favor of 
their opinions? 6. What hy the latter? 



tpip: milky way. 189 

results. Witli the optical facilities which we now 
possess, we are enabled not only to descry the nebulae, 
but in many instances to penetrate beyond their filmy 
dimness that first attracts our notice, and fathom their 
depths and behold their glory. Many of these objects 
may be seen with instruments of moderate powers; and 
bv constautly increasiug their powers they manifest 
themselves more distinctly, till thousands of stars, of 
ahnost every class and color, burst forth into view, appa- 
rently in such proximity as to appear like diamond 
points joined together at insensible distances. 

7. Some of the nebulse offer such phenomena when 
they are. made the objects of special investigation, whilst 
others have resisted every eftbrt which has been made 
to resolve them. The more closely they have been 
examined by a succession of instruments of increased 
penetrating powers, they exhibit more and more the 
appearance of what, in all probability, the great majority 
of them are, — clusters of stars. 



SECTION LXXX. 

m^z piling Mag. 

1. The Milky Way has to the naked eye a nebulous 
appearance (see Fig. 48) ; but if a telescope of high 
power is directed towards it on a cloudless night, the 
nebulosity disappears, and thousands of very small stars, 
otherwise invisible, will, even in a few moments, be 
presented to view. 

2. In this dim luminous zone one cluster of stars is 
crowded upon another so compactly that their parts 
appear to constitute as it were a great highway around 

Questions. — Which theory accords best with what is known ? 
7. Have some of the nebulae resisted every effort to resolve them ? 

Sec. LXXX. — 1. How does the Milky Way appear when viewed 
by the telescope? 2. Are the stars that compose it very numerous 
and apparently near together? 



190 MAGELLAN CLOUDS. 

the heavens, paved with stars. These clusters are buried 
deep in space; and were it not for the artificial multipli- 
cation of the powers of vision, their existence would 
never have been indicated by that whitish hazy arch of 
light which we are permitted to observe. 

3. The galaxy which they form, together with every 
other heavenly object similar in appearance in point of 
light, Avould have remained a hidden mystery, and those 
innumerable solar universes, far beyond the limits of 
our own, would have never revealed to us their existence. 
Not only would we be destitute of a knowledge of them, 
but also of all the peculiarities whereby they may be 
distinguished. Some of them are exceedingly irregular 
in their forms, and present outlines very indefinite and 
confused, like the Magellanic Clouds, as they are called, 
which are situated in the southern hemisphere. 



SECTION LXXXI. 



1. The two Magellan Clouds, or, more properly, the 
two patches of light which are known by that name, 
are situated in the southern hemisphere, and cover an 
area of about fifty degrees. These objects differ in size, 
are oval in their general form, and are visible, when 
above the horizon, to the naked eye. The smallest is 
the most brilliant, and in appearance both of them 
resemble the Milky Way. Some astronomers consider 
these objects as off-shoots or branches of the galaxy, and 
it is possible that they are : still, we may regard them as 
constituting a class differing somewhat from it in con- 

QuESTiONS, — 3. Are we indebted to the telescope for all of these 
discoveries ? 

Sec. LXXXI. — 1. Where are the Magellan Clouds situated ? How 
much of the heavens do they occupy? What is the form of these 
objects? Are they visible to the naked eye? Which is the most 
brilliant ? What do they resemble ? In what respect do they difier 
from it ? 



NEBULiE PROPER. 191 

sequence of the inherent diversity of the objects that 
compose them, their irregularity of form and indefinite- 
ness of outline. 

2. When either of these clouds is examined through 
telescopes of great power, they disclose to the observer 
single stars, clusters of stars, and nebulre. In general 
the clusters appear globular in form, and the stars that 
compose them, in certain instances, appear crowded upon 
each other towards their centres. The nebulae that have 
been discovered in these clouds are of nearly every variety 
of formation found elsewhere in the heavens, and some 
are of unusual forms, which are nowhere else discover- 
able in the universe. Few of these unsymmetrical 
figures resemble each other ; neither are they uniformly 
distributed in the celestial regions which they occupy. 

3. About three hundred and twenty clusters and 
nebulae have been discovered in the larger cloud, and 
about forty of both kinds have been observed in the 
smaller one. If we regard either of these clouds as 
single bodies, they are less rich in stars and less con- 
densed towards the interior than any known class of 
stellar clusters or nebulae, except those known as the 
nebulae proper. 



SECTION LXXXII. 

^ebulae proper. 

1. Of this class there is a great variety. Some of 
them appear to be formed of little flocky or fleecy 
scrolls of matter, slightly wrapped together, like wisps 
of cloud, which are as it were casually distributed at no 



Questions. — 2, Of what are these objects composed ? What forma 
have the clusters? What is said of the nebulae? 3. How many 
clusters and nebulae are in the larger one? How many in the 
smaller ? What is said of these clouds as single bodies ? 

Sec. LXXXII. — 1. What is the appearance of some of the npbulse 



192 NEBULA PROPER. 

great distance from each other. Others appear more 
uniform in their character, and have outlines which are 
definite and regular. They manifest no signs of resolva- 
bility, neither do they indicate that they are any thing 
except self-luminous phosphorescent patches of gaseous 




Fig. 49. 

vapor suspended against the black groundwork of the 
heavens. (See Fig. 49.) 

2. In like manner is it with many of those that pre- 
sent a more definite and substantial appearance. Not- 
withstanding some of them are elliptical, and increase 
by insensible gradations of brightness up towards the 
centre, yet" they do not appear as if they were clusters 



Questions. — What of others ? Can they be resolved ? Wliat do 
they appear to be? 2. What is said of those that are somewhat 
elliptical in form? 



ANNULAE NEBULA. 193 

of Stars, but only nebulae in a high state of conden- 
sation. So also do those that are spindle-shaped resist 
every effort that has been made to resolve them into 
stars. 

3. Some astronomers suppose that these objects are 
composed alone of self-luminous, nebulous matter ; whilst 
others think it probable that their form and appearance 
result from the oblique direction in which a ring com- 
posed of clusters of stars sunk deep in space may be 
seen, as such objects are known to exist. 



SECTION LXXXIII. 

giunular ^tbulac. 

1. Only nine of these nebulse have been discovered 
in the heavens, and every effort which was made till 
recently proved ineffectual in revealing their nature and 
character. Sir William Herschel had a knowledge of 
their existence, and some of them have received special 
attention by Lord John Rosse, with the aid of his great 
reflector. 

2. The most celebrated one in the northern hemi- 
sphere is somewhat elliptical in form, and in a number 
of places around the ring it has been resolved into lumi- 
nous points resembling stars. (See Fig. 50.) With an 
instrument of high power its outer margin is rendered 
irregular by the development of nebulous appendages; 
and the interior of the ring is slightly illuminated by 
a faint, hazy light, gradually decreasing towards the 
centre. 

3. Others of this class are circular, and are well 



Questions. — 3. Are all of these objects self-luminous? 

Sec. LXXXIII. — 1. How many annular nebulse have been dis-i 
covered? What astronomers examined them? 2. Where is the 
most celebrated one located ? What is its form ? Have portions of it 
been resolved ? What is said of its outer and inner margins ? 3. 
What is said of other nebulse of this class ? 

17 



194 



ANNULAK NEBULA. 




Fiff. 50. 



STELLAR NEBULA, AND NEBULOUS STARS. 195 

defined both without and within. They are in a great 
measure destitute of nebulous offshoots around their 
contours, and resemble a ring flattened in the direction 
of its plane, having no stars or nebulosity in the interior. 
They manifest no signs of resolvability, probably not 
because they are entirely nebulous, but because the 
annular congeries of stars of which they may be com- 
posed, contain stellar points so minute, by being so far 
away, as not to afford singly sufficient light to make 
an impression on the retina of the eye. 



SECTION LXXXIV. 

Stellar glebulsc, anb ^ebulous ^tars* 

1. Besides the annular nebulse, which we have just 
described, there are in the heavens objects equally if 
not more mysterious, known as stellar nebulae, and 
nebulous stars. (See Fig. 51.) These objects differ 
somewhat in their outlines and general appearance. 
Sometimes the nebulous fluid is apparently drifted to 
one side of the star. Again it appears in globular form, 
gradually condensing from the circumference to the 
centre. In some the condensation is more sudden near 
the centre, presenting the appearance of a dull and 
blotted star. 

2. In others there may be found the singularly 
beautiful and striking phenomenon of a sharp and bril- 
liant star surrounded by a perfectly circular disc or 
atmosphere of faint light, in some cases dying away on 
all sides by insensible gradations, and in others termi- 
nating almost suddenly. 

3. These wonderful objects naturally lead the mind 



Questions. — Have they been resolved into stars ? What is the 
probable reason why they have not been ? 

Sec. LXXIY. — 1. What objects are noticed in this section ? What 
is said of some of these objects? 2. What of others? 3. Are these 
objects delinitelv understood? 



196 STELLAR Is^EBUL^, AND NEBULOUS STARS. 




Fig. 51. 



PLANETARY NEBULA. 197 

to inquiry and speculation. Were they originally pri- 
mordial nebulous matter, and are they gradually con- 
densing, and thereby arriving more and more at the per- 
fection of stars or suns? Are their atmospheres self-lumi- 
nous, or illuminated by the central body ? Are they at 
an equal distance from us with other stars, and will they 
ever lose their nebulosity ? These questions challenge 
the judgment; and with regard to being able to solve 
them, science has hitherto been silent. 



SECTION LXXXV. 

'^lanctarii ^cbulij. 

1. In like manner is it also with those objects in the 
heavens known as the planetary nebulae. They present 
the most striking resemblance to planetary discs ; and, 
whilst the greater number of them appear perfectly 
globular and sharply defined, yet there are others indis- 
tinct and vaporous at their margins. 

2. They are of various colors, being usually uniform 
in their light, though in certain instances it is slightly 
curdled or mottled. From experiments which have 
been made on light emitted from a disc and that emitted 
from a self-luminous point, it is not improbable that 
they are very remote nebulous stars, in which the differ- 
ence between the central body and the surrounding 
nebulous covering cannot be detected by the telescope. 

3. A few of these objects have been partially resolved, 
so that in some, one star appears in the centre, and in 
others, two near too;ether, with a rino; of hazv lio:ht, 
emitted, it may be, by a great annular cluster of suns 
concealed in the infinite depths of space. (See Fig. 52.) 

Questions. — Do we know much concerning their physical consti- 
tutions? 

Sec. LXXXV. — 1. What are the planetary nebulae? What are 
their forms ? 2. Are they all of one color ? What are these bodies ? 
3. Do stars appear in the centre of any of them ? 



PLANETARY NEBULiE. 




GLOBULAR NEBULA OK CLUSTERS. 199 

But, whatever may be their nature and the nature of 
the nebulous stars, they must be of enormous magni- 
tude ; and, granting that they are at an equal distance 
from us with the fixed stars, their real dimensions must 
be such as would fill the whole orbit of one of the 
outermost planets of the solar system. Such are some 
of the wonders that are scattered over the boundless 
field of space; yet there are other objects more wonderful. 



SECTION LXXXVI. 

dilobular J^cbulae or Chtslers. 

1. There are a great number of globular nebulse, 
promiscuously distributed in the heavens, which ^ when 
examined with instruments of great power, are per- 
ceived to consist generally of stars crowded together 
so as to have almost a definite outline and to run uj) 
almost to a blaze of light in the centre. Many of 
them present a figure exactly round, and thereby convey 
the idea of a perfectly spherical spacefilled full of stars. 

2. Each appears insulated in the heavens, and to con- 
stitute in itself a family or society apart from the rest 
and subject only to its own internal laws. Others of 
them are not so regular in their outlines, in consequence 
of the paucity of stars in the outer parts, and, instead 
of exhibiting an ordinary stellar light, they are filled 
around their centres with rose-colored light, exhibiting 
an imposing contrast with the light of the exterior stars 
by which they are surrounded. 

Questions. — Are these objects very large? Admitting that they 
are as distant as the fixed stars, what would be the diameter of some 
of them ? 

Sec. LXXXVI. — 1. Of what are the globular nebulae generally 
composed ? How do they appear near their centres ? 2. Are they 
apparently insulated in the heavens ? Are their component parts 
subject to physical laws? Are these globular nebulse ever irregular 
in their outlines? What is the color of some of them at their 
centres ? 



200 GLOBULAR NEBULJS OR CLUSTERS. 

3. The stars in these clusters are very numerous, and 
are not to be reckoned by hundreds, but by thousands, 
apparently wedged in and projected on one another, so 
tliat the eye becomes enchanted by observing them, and 
the mind becomes bewildered in their contemplation. 
(See Fig. 53.) These clusters vary in size; and from 




Fig. 53. 

their forms it may be legitimately inferred that they are 
controlled in all their parts by the same laws which 
control all systems which pervade the universal heavens. 
The order and harmony that prevail in them assure us 

Questions. — 3. Are they composed of many stars? Do they ap- 
pear crowded on one another? Do these chisters vary in size? Are 
the existence and motions of the component parts of these systems 
tlie resuh of chance? 



SPIRAL NEBULA. 201 

of the fact that their arrangement is not the result of 
chance, but that they are constantly made to feel the 
power of Him who has marshalled out the stars and 
called them by their names. 

4. The law of gravitation is wherever matter exists, 
and has manifested itself not only in the peculiar forma- 
tion of worlds, but also in the formation and perpetuity 
of those celestial scheme? and systems, a perfect know- 
ledge of which is possessed only by Him who is the 
Maker of them all. 



SECTION LXXXVIL 

spiral '§thnlnz. 

1. The spiral nebulae are probably the most remark- 
able objects in ,the heavens, both on account of their 
singular configurations and also the transformative 
effects produced on them by the telescope. Under its 
gaze some of them appear to change into heliacal tAvisted 
coils, or self-luminous spirals, Avhose convolutions appear 
unequal both in their centres and outwards, and are 
prolonged at either extremity into dense, globular knots. 
(See Fig. 54.) Others also pass through many phases 
when placed under the increased power of optical instru- 
ments, until when in their last stages of development 
they resemble a crown of luminous hair. 

2. These objects are few in number ; and the inquiry 
now, no doubt, suggests itself. What can they be? 
Are they systems of stars so far away and so closely 
crowded together as to appear like a single body ? or are 
some of them double systems of suns, held in dynamical 
equilibrium by other cosmical systems, equal if not. 
superior to themselves ? Do their component parts 

Questions. — 4. What law has manifested itself in their formation 
and government ? 

Sec. LXXXVII. — 1. Describe the spiral nebulae. 2. Are the 
spiral nebulae numerous? Are they systems of stars or suns? 



202 



SPIRAL NEBULA. 




Fig. 54. 



STRUCTURE AND HARMONY OF THE UNIVERSE. 203 

obey equally the centripetal and centrifugal forces, or 
are they in a constant state of collapse? Does their 
light represent their true form, or is it modified, in this 
respect, by any unknown influence? 

3. In view of such unsolved problems as these, are 
we not ready, with the inspired penman, to exclaim, 
" Great and marvellous are thy works, Lord God 
Almighty," and does not a sense of our limited knowledge 
and of thine infinite greatness and the greatness of thy 
works constrain us to add. Truly, not only Thyself, but 
they also, are past finding out ? 



SECTION LXXXVIIL 

Structure anb parmoitj) of tbe ^nibersr. 

1. Having now presented very briefly, and in a mea- 
sure separately and consecutively, some of the most 
prominent features and phenomena of the sun, planets, 
satellites, comets, nebulae, and fixed stars, let us take a 
glance at them all as they loom up in beauty and gran- 
deur before the intellectual vision. Go, in imagination, 
to yonder sun which is the author of light and life to 
our earth, and contemplate his might and his magnitude. 
He is hundreds of times greater than the aggregate of 
all his subordinates that play annually around him, and, 
according to an unerring providence, he controls them 
with a power absolute as despotism itself. No planet 
wanders out of its appointed way that he does not 
reclaim. No satellite deviates from its appointed course 
that he does not correct. No waywardness in the 
wandering comet is manifested that he does not restrain. 



Questions. — Are they subject to physical laws with which we are 
familiar? Are they self-luminous? 3. "What should our limited 
knowledge and the greatness of nature's works teach us ? 

Sec. LXXXVIII.— 1. On what does this section treat? Does the 
sun control the planets and other bodies that revolye around him? 



204 STRUCTURE AND HARMONY OF THE UNIVERSE. 

Implicit obedience is rendered by his subjects, infinitely 
more than unto a monarch on a throne. 

2. But is the sun, with all his might, and grandeur, 
and glory, the great masterpiece of this boundless empire 
which we behold around us ? Is he the only physical 
potentate that bears rule in the heavens? or is he himself 
not a subject? yea, a subject! There is a higher and 
mightier force than what he exerts, probably in the 
Pleiades, to which he with all his glorious cortege 
renders unremitted homage. With his retinue of de- 
pendent worlds he is hastening on with lightning speed 
to make his annual journey of more than eighteen mil- 
lions of our years around the centre of this '^island 
universe'' of suns, of which this planetary system of 
ours is only an infinitesimal part. 

3. The stars known by the name of fixed, and it may 
be all centres of subordinate systems, are paying obei- 
sance also to this central force. It controls their motions 
according to their distances and their masses, and as a 
king of kings it sways universal empire over thousands 
of systems that roll majestically over the illimitable 
field of space. 

4. But is this mighty central force, even, in this stellar 
universe of ours, the universal governor of all ? Others 
may exist differing in office almost as much as finite 
differs from infinite. They may ascend by gradations 
in their functions, increasing in power and control in 
proportion to the increase of the number and magnitude 
of the suns and schemes and systems and firmaments 
that are their subjects. ^They may tend upwards in 
office and effect towards the eternal throne itself, till all 
created island universes, with all of their constituent 
parts, shall in their respective cycles circumscribe tlie 



Questions. — 2, Is the sun the centre, of the universe ? Does the 
sun revolve around a centre? Where is that centre? How long is 
he making one revolution ? 3. Do the stars also revolve around this 
centre ? Do whole stellar systems revolve around this centre ? 4. 
May there be other centres of motion superior to the sun, or that in 
the Pleiades? 



STIJUCTUJJE AND HARMONY OF THE UNIVERSE. 205 

highest ; yet above them all there is an independent and 
imiimtable eentre, around which they all move in har- 
mony, an omnipotent Sun, from whom they all received 
their existence^ an eternal Sun, to whom they all belong, 
and an everlasting central Sun, to whom they will for- 
ever be subordinate, — even the Sun of righteousness. 

Questions. — Is there any living centre to which all others are 
subordinate ? Who is He ? 



18 



EXPLANATION 



OF 



ASTRONOMICAL TERMS AND PHRASES. 



Aberration of Stars- Their apparent change of place caused by the 

velocity of light and the motion of the earth in its orbit. 
Absorbent Media — Substances which absorb the rays of light and heat. 
Acceleration— An increase of the motion of a moving body. 
Acronycal — Rising or setting with the sun. 
Acute Angle— One less than a right angle. 
Aeriform — Having the form of air. 
Aerolite — A meteoric substance. 

Altitude of a heavenly body — Its height above the horizon. 
Amphiscii — Inhabitants of the torrid zone. 
Amplitude — The distance which heavenly bodies rise or set north or 

south from the east or west point of the horizon. 
Analemma — A figure drawn on the artificial globe, from one tropic to the 

other, on which is marked the sun's declination for each day in the 

year. 
Angle — The space where two lines meet. ^ 

Angular Distance — The distance between certain objects which is repre- 
sented by the angle formed by straight lines drawn to them from a 

given point. 
Annual Equation — A periodical inequality in the motion of a planet, 

or the moon going through its changes during a year. 
Annual Parallax — The difference of the position of a body as seen from 

opposite points in the earth's orbit. 
Annual Revolution of tbe Earth — Her yearly revolution round the sun. 
Annular — Having the form of a ring. 
Anomalistic Year — The time that the earth is in moving from perihelion 

to perihelion again. 
Anomaly — The sun's angular distance from the apogee, or the earth's from 

aphelion. 
Antarctic — Southern. 
Antarctic Circle — A circle 23° 28' distant at all points from the south 

pole. 



334 EXPLANATION OF TERMS AND PHRASES. 

Antipodes — Persons who live on the sides of the earth directly opposite. 
Antoeci — Persons who live on the same side of the earth, equally distant 

north and south from the equator. 
Aphelion — That point in the orbit of a planet which is farthest from the sun. 
Apogee — That point in the orbit of the moon farthest distant from the 

earth. 
Apparent Diameter — The distance from one side of a heavenly body to 

the other, as seen from the earth. 
Apsides — The points in an orbit at the greatest and least distance from 

the centre around which a heavenly body revolves. 
Aquarius — A sign of the zodiac. 
Arc — A part of the circumference of a circle. 
Arctic — Northern. 

Arctic Circle — A circle 23° 28' distant at all points from the north pole. 
Areas — Portions of the plane embraced within the orbit of a celestial body. 
Aries — A sign of the zodiac. 
Ascending Node — The point on the ecliptic where the moon passes north 

of it. 
Ascensional Difference — The difference between oblique and right ascen- 
sion. 
Aspect — The relative appearance of heavenly bodies in relation to posi- 
tion, angular distance, <fec. 
Asteroids — Small planets which revolve around the sun between the orbits 

of i\Iars and Jupiter. 
Astronomical Time — Time reckoned from noon of one day till noon of 

the next. 
Atmosphere — A transparent, elastic, fluid substance which surrounds the 

whole earth. 
Attraction — The tendency which all bodies have of coming together. 
Aurora — The morning twilight. 
Aurora Borealis — The Northern Light. 
Austral — Southern. 
Autumnal Equinox— That point in the equinoctial through which the 

earth passes in September, when day and night are equal. 
Axes of an Ellipse — The lines that cross each other at right angles at the 

centre of the figure and divide it into four equal parts. 
Axis of a Great Circle — The straight line that passes through its centre 

perpendicular to its plane. 
Axis of Rotation — An imaginary line around which a revolving body 

turns. 
Azimuth — The difference of distance between the amplitude of heavenly 

bodies and 90° measured from north and south. 

Binary System — Two stars revolving around each other. 



EXPLANATION OF TERMS AND PHRASES. 335 

Bissextile or Leap Year — Every fourth year, in which February ha& 
twenty-nine days. 

Calendar — An almanac or record of the divisions of time. 

Calendar Months — The months as marlied in the calendar. 

Cancer — A sign of the zodiac, 

Capricorn — A sign of the zodiac. 

Cardinal Points — East, west, north, and south. 

Celestial Horizon — An imaginary circle where the plane of the rational 

horizon touches the heavens. 
Celestial Sphere — The apparent concave surface of the heavens. 
Centrifugal Force — A force which causes a body to move forward in a 

straight line. 
Centripetal Force— The force which causes a revolving body to tend 

towards its centre of motion. 
Chord — A straight line extending to each end of an arc. 
Circle — A plane figure bounded by a circumference ■equally distant at all 

points from its centre. 
Circle of Illumination — The circle that separates the unenlightened por- 
tion of the earth from what is enlightened. 
Circle of Perpetual Apparition — The boundary of that space around the 

elevated pole where the stars never set. 
Circle of Perpetual Occultation — The boundary of that space around the 

depressed pole within which the stars never rise. 
Circles of Daily Motion — Circles described by the heavenly bodies in 

their apparent daily motion from east to west. 
Circumference— The curved line that bounds a circle. 
Circumpolar Stars — Stars around the pole that do not sink below the 

horizon. 
Colures — Meridians which pass through the equinoctial and solstitial 

points of the ecliptic. 
Comets — Rare bodies that revolve around the sun. 
Complement of an Arc— What it wants of 90°. 
Concave — Hollow surface of a sphere. 
Concentric Circles — Circles having a comuion centre. 
Cone — A solid with a circular base and tapering up to a point. 
Conjugate Diameter— The shortest diameter of an ellipse. 
Conjunction — A planet is in conjunction when it is iii a line with the earth 

and the sun. 
Constellations — Groups of stars. 
Convex — Round, like the surface of a ball. 
Corona — A crown. 

Cosmical — ^Pertaining to heavenly bodies. 
Culminate — To arrive at the highest point attainable in the heavens. 



o-JG EXPLAXATIOX OF TERMS AND PHRASES. 

Culmination — To pass the highest point attainable in the heavens. 

Cusps — -Extremities of the moon's crescent. 

Cycle of a Planet — A period in which a planet passes through its various 

positions in relation to the sun and earth. 
Cycle of the Moon, or Metonic Cycle — A period of nineteen years, when 

the changes of the moon return to the same days of the month. 
Cycle of the Sun — A period of twenty-eight years, when the same days 

of the month return to the same days of the week. 

Declination — The distance of a heavenly body north or south from the 

equinoctial. 
Degree — The three-hundred-and-sixtieth part of the circumference of a 

circle. 
Descending Node — The point on the ecliptic where the moon passes south 

of it. 
Dial — An instrument that shows the time of the day by the shadow of its 

gnomon, or hand. 
Diameter — A straight line that passes through the centre of a figure and 

terminates at its surface. 
Diameter of the Celestial Equator — The diameter of the earth's equator 

extended to the starry heavens. 
Dichotomized — Divided into equal parts. 

Digit — The one-twelfth part of the a})parent diameter of the sun or moon. 
Dionysian Period — A period found by multiplying the cycles of the sun 

and moon together, and is equal to five hundred and thirty-two years. 
Direct Motion — Motion eastward. 
Disc— The apparent face of a body. 
Diurnal Arc — The arc described by a heavenly body from the time it 

rises till it sets. 
Diurnal Parallax — The difference between the apparent and true place of 

a body. 
Diurnal Revolution — Daily motion of a body on its axis. 
Dominical Letter — The letter in the calendar representing Sunday. 
Dominical Letters — The first seven letters of the alphabet, used to repre- 
sent the first seven days of the year. 

Earth — The sphere on which wc live. 

East — The direction in which the sun rises when the days and nights are 

equal. 
Eccentric — Out of the centre. 
Eccentric Circles — Circles that are wholly or partially within each other 

with different centres. 
Eccentricity — Distance from the centre of an ellipse to either focus. 
Ecliptic — The apparent annual pathway of the sun among the stars. 



EXPLANATION OF TERMS A^1J> PHKASES. 337 

Element — A fundamental principle, or cunstitucnt part. 

Ellipse — An oval figure. 

Elongation — The angular distance of certain heavenly bodies from the 

sun. 
Emersion — Reappearing. 

Epact— The age of the moon at the beginning of the year. 
Epicycles — Curves described by the point of one circle revolving upon 

another. 
Epoch — A particular time or period. 
Equation of Time — Time to be added to or subtracted from sun time to 

find mean time. 
Equator — An imaginary great circle passing round the earth east and 

west, everywhere equally distant from the poles. 
Equatorial Diameter — An imaginary line passing through the centre of 

a body at right angles to the polar diameter. 
Equinoctial — An imaginary circle where the plane of the equator touches 

the heavens. 
Equinoctial Points — Points where the equinoctial cuts the ecliptic. 
Equinox— Day and night equal in length. 
Evection — A periodic inequality in the motion of the moon. 

Firmament— The starry heavens. 

First Meridian — That point from which longitude is reckoned. 

Fixed Stars — Self-luminous heavenly bodies which appear to retain their 

relative positions. 
Focus — The point to which rays converge, or one of the elements of an 

ellipse. 

Galaxy— The Milky Way. 

Geocentric — Having the earth as a centre. 

Gibbous — The moon is gibbons in appearance when more than half, anO 

not entirely, full. 
Globe — A sphere. 
Golden Number — The number of years in the cycle of the moon since the 

epact was nothing. 
Graduated Circle — An artificial circle divided into parts, called degrees, 

minutes, and seconds. 
Gravity — The force which draws all bodies towards each other. 
Greatest Elongation — The greatest angular distance of a planet from the 

sun. 

Harvest Moon — The full moon at or near the autumnal equinox. 
Heliacal — Pertaining to stars that rise a little before the sun rises, or set 
a little after he sets. 

29 



338 EXPLANATION OF TERMS AND PHRASES. 

Heliocentric — Having the sun as the centre. 

Heliometer — A sun-measurer. 

Hemisphere — Half a sphere. 

Heteroscii — The inhabitants of the temperate zones. 

Higher Apsis — That point in an orbit at the greatest distance from the 

centre of motion. 
Horizon, or Sensible Horizon — The circle where the sky and earth appear 

to meet. 
Horizontal — Parallel to the horizon. 

Horizontal Parallax — The diurnal parallax of a body at the horizon. 
Hour Circle — A small circle near the north pole of an artificial globe, 

having marked on it the hours of the day. 

Immersion — Disappearing. 

Inferior Conjunction — A body is in inferior conjunction when it is between 

the earth and the sun, in a line with both. 
Inferior Planets — Those that are always nearer the sun than the earth. 
Intercalation — Inserting extra time into some of the divisions of time. 

Julian Period — A period of seven thousand nine hundred and eighty 
years, which is the product of the cycles of the sun and moon and 
the Roman indiction. 

Julian Year — A period of three hundred and sixty -five and one-quarter 
days. 

Latitude — Distance north or south of the equator. 

Latitude in the Heavens — Distance north or south from the ecliptic. 

Leap-Year — Every fourth year in which one day is added. 

Leo — A sign of the zodiac. 

Libra — A sign of the zodiac. 

Librations of the Moon — Periodic oscillations of her four cardinal limbs. 

Limb — A portion of the curved edge of the sun or moon's disc. 

"'^iine of CoUimation — The imaginary line in a telescope which joins the 

centres of the object- and eye-glasses. 
Line of the Apsides — The line that unites the two apsides. 
Line of the Nodes — The imaginary line that unites the ascending and 

descending nodes of the moon's orbit. 
Longitude — Distance east or west of any given meridian, measured on 

the equator. 
Longitude in the Heavens — Distance eastward from the first point of 

Aries, measured on the ecliptic. 
Lower Apsis — That point in an orbit at the least distance from the centre 

of motion. 
Lunar — Pertaining to the moon. 



EXPLAXATIOX OF TERMS AND PHRASES. 839 

Lunar Distance — Distance of the moon from a given celestial object. 
Lunar Month — The time from one change of the moon till the next. 
Lunation — Average length- of the lunar month. 

Mass — Quantity of matter in a body. 

Mean Motion — Average motion. 

Medium — In astronomy, any substance through which light aiul heat will 

pa.-is freely. 
Meridian — \ great circle passing through the poles. 
Meteor — .A. rare substance that becomes luminous as it passes through the 

air. 
Meteorite—A dense substance which sometimes falls upon the earth. 
Minute — The .sixtieth part of an hour or degree. 

Nadir — The lov.er pole of the horizon. 

Neap Tide— The less tide. 

Nebulae — A name given to celestial objects that have the appearance nf 
luminous clouds or mist. 

New Style — Tlie present mode of reckoning time, established by Gregory 
XIIL 

Nocturnal Arc — The arc described by a heavenly body during the night. 

Nodes — The points of the moon's orbit where it crosses the ecliptic. 

Nonagesimal Degree — The highest point of the ecliptic above the horizon. 

Nucleus — The densest part. 

Nucleus of a Comet — The head. 

Nutation — An oscillating motion of the earth's axis, caused by the attrac- 
tion of the sun and moon on the excess of matter at the equator. 

Oblate Spheroid — A sphere flattened on the opposite side?. 

Oblique — Deviating from a perpendicular. 

Oblique Ascension — That point in the equinoctial which rises with a body 

in an oblique sphere. 
Oblique Descension — That point in the equinoctial which sets with a body 

in an oblique sphere. 
Oblique Sphere — That on which the circles of daily motion are oblique to 

the horizon. 
Obliquity of the Ecliptic— The difference between the plane of the 

equinoctial and the plane of the ecliptic, marked by degrees. 
Obtuse Angle — One greater than a right angle. 
Occidental — Westward. 

Occultation — The eclipse of a star by another heavenly body. 
Octant — The e'ghth part of a circle. 

Old Style — The mode of reckoning time which Gregory XIII. reformed. 
Opaque — Dark, not luminous. 



340 EXPLANATION OF TERMS AND PHRASES. 

Opposition — When two bodies are on the opposite sides of another body, 

they are in opposition. 
Orbit — The line which a body describes as it revolves around another 

body. 
Orbital Motion — The motion of a heavenly body around its centre of 

motion. 

Parallactic Angle — The angle formed by observing a body from two 

different points. 
Parallactic Motion — Angular motion which is perceptible. 
Parallax — The apparent difference in position of a body when viewed from 

different points. 
Parallel Lines — Lines equally distant from each other at all points. 
Parallels of Altitude — Small circles parallel to the horizon. 
Parallels of Declination — Small circles parallel to the equinoctial. 
Parallels of Latitude — Small circles parallel to the equator. 
Parallel Sphere— That in which all the circles of daily motion are parallel 

to the horizon. 
Penumbra— A dim shadow. 

Perigee — The nearest point of the moon's orbit to the earth. • 
Perihelion — The nearest point to the sun of the orbit of a planet. 
Periodic Inequality — Irregularity in the motion of a heavenly body. 
Periodic Time — The time required for a body to revolve around its centre 

of motion. 
Perioeci — Persons who live equidistant from the equator, on opposite sides 

of the earth. 
Periscii — Persons who live in the frigid zones. 
Perturbations — Deviations in the direction of moving bodies. 
Phases — An increase or diminution in quantity of the enlightened surface 

of a heavenly body when viewed from the earth. 
Phenomena — Various manifestations of natural objects. 
Pisces — A sign of the zodiac. 
Plane — Surface without thickness. 
Plane of a Circle — Surface extended indefinitely, in which the whole 

circle is contained. 
Planet — An opaque body that has the sun for its centre of motion. 
Pleiades — Seven stars in the constellation Taurus. 
Pointers — Two stars in the Great Bear. 
Polar Circles — Circles twenty-three and one-half degrees at all points 

from the poles. 
Polar Diameter — An imaginary line about which a body rotates. 
Polar Distance — Angular distance from the poles. 
Polar Star — A star near the north pole of the heavens, of the second 

magnitude. 



EXPLANATION OF TERMS AND PHRASES. 341 

Poles, Celestial — Points in the heavens where the earth's axis would touch 

if produced. 
Poles, Terrestrial — Extremities of the earth's axis. 
Precession, of Equinoxes — A westward motion of the equinoctial points 

on the ecliptic. 
Prime Vertical Circle — The circle which passes through th« east and west 

points of the horizon. 
Prolate Spheroid — An elongated sphere. 

Quadrant — Quarter of a circle. An insti'umcnt by which angles are 

measured. 
Quadrature — The moon is in quadrature when half full, or quarter of a 

circle from the sun. 
Quadrilateral — A geometrical tigure with four sides. 
Quartile — Ninety degrees apart. 
Quiescent — At rest. 

Radiate — To emit rays. 

Radius — A straight line drawn from the centre of a circle to its circum- 
ference. 

Radius-Vector — An imaginary line extending to the sun from the earth. 

Rational Horizon — The circumference of a plane parallel to the plane of 
the sensible horizon, which divides the earth into two hemispheres, 
the upper and lower. 

Rectilinear — Straight-lined. 

Reflection — Throwing oflF the rays of light or heat. 

Refraction — The breaking ef rays of light as they pass through media 
that differ in density. 

Repulsion — The 25roperty which some bodies have which has a tendency 
to separate them. 

Resisting Medium — An exceedingly subtle fluid, which is supposed by 
some to pervade all space. 

Retrograde Motion — Apparent motion from east to west. 

Revolution of a Body — Motion of a body till it returns to the point that 
it left. 

Right Angle— One equal to 90°. 

Right Ascension — Distance measured on the equinoctial from the first 
point of Aries. 

Right Line— A straight line. 

Right Sphere — One in relation to which all the apparent daily revo- 
lutions of the heavenly bodies are in circles perpendicular to the 
horizon. 

Roman Indiction — A period of fifteen years. 

Rotation — Motion of a body on its axis. 

29* 



342 EXPLANATION OF TERMS AND PHRASES. 

Satellite — The moon. A secondary planet. 

Scorpio — A sign of the zodiac. 

Secondary Circles — Circles that are perpendicular to others on which 

they depend. 
Secondary Planets — Those that revolve around the primary planets. 
Sector of a Circle — Area enclosed by an arc and two radii, which is less 

than a semicircle. 
Secular Inequalities — Variations in the motion of heavenly bodies, that 

have long periods. 
Segment — Part of a circle enclosed by an arc and chord. 
Semicircle— Half a circle. 
Sextant— The sixth part of a circle. 
Sidereal — Pertaining to the stars. 
Sidereal Day — The time from one transit of a star till the next over the 

samL^ meridian, caused by the rotation of the earth on its axis. 
Sidereal Year — The length of time taken by the earth to perform one 

entire revolution in her orbit. 
Sign — The twelfth pai-t of a circle. 
Solar — Pertaining to the sun. 

Solar Day — The time from noon of one day till noon of the next. 
Solar System — The sun, planets, moons, and comets, in their natural 

order. 
Solstitial Points — Those two points on the ecliptic where the sun is at 

his greatest distance north and south of the equator. 
Sothis, or Sirius— The Dog-star. 
Sphere— A globe or ball. 
Spherical — In the shape of a sphere. 
Spheroid — Resembling a sphere. 
Spring Tide — The greater tide. 
Stationary — Occupying one position. 
Summer Solstice — That point on the ecliptic where the sun is at his 

greatest distance north of the equator. 
Superior Conjunction^-A body is in superior conjunction when it is out- 
side of the earth from the sun, and in a line with both. 
Superior Planets — Those that are at a greater distance from the sun than 

the earth. 
Synodic Month — The time from new moon till new moon again. 
Syzygies — Points in the moon's orbit where she changes and fulls. 

Taurus — A sign of the zodiac. 

Terminator — A line that divides between the illuminated and shaded 

portions of a heavenly body. 
Tide — The flux and reflux of the waters of the ocean. 
Transit — Passinsr ever. 



EXPLANATI<:)N OF TEEMS AND PHEASES. 343 

Transit of a Star — Its passage over any given meridian. 

Transit of Mercury — Mercury passing over the sun's disc. 

Transit of Venus — Venus passing over the sun's disc. 

Transverse Diameter — The longest diameter of an ellipse. 

Triangle — A figure bounded by three sides. 

Tropic of Cancer — A small circle distant north from the equator 23° 28' 

at all points. 
Tropic of Capricorn — A small circle distant south from the equator 23° 

28' at all points. 
Tropical or Solar Year — The time that it requires the earth to depart and 

return to the same solstice. 
True Place of a Planet — The place where it would appear if seen from 

the centre of the earth. 
Twilight — Refracted and reflected light visible before sunrise and after 

sunset. 

Umbra — A dark shadow. 

Universe — A large cluster of stars. The whole material creation. 

Uranography — A description of the heavens. 

Vernal Equinox — That point in the equinoctial through which the earth 

passes in March, when day and night are equal 
Vertex — The top. 

Vertical — Towards the centre of the earth. 
Vertical Circle — A circle in a vertical plane. 
Vertical Plane — A plane coinciding with a vertical line perpendicular to 

the horizon. 
Virgo — A sign of the zodiac. 

Waning — Declining, or decreasing. 
Wind — Air in motion. 

Winter Solstice — That point on the ecliptic where the sun is at his 
greatest distance south of the equator. 

Zenith Distance— The angular distance of a heavenly body from the 

zenith. 
Zodiac — A zone extending clear round the heavens, 8° on either side of 

the ecliptic, containing twelve signs. 
Zone — A portion of the earth's surface, limited by circles parallel to the 

equator. 



THE END. 



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